Update README.md for Project Euler (#3256)

* Update README.md for Project Euler

* Add link to solution template

* Add newlines for better separation

* Add __name__ == __main__ block in template

* Apply suggestions from code review
Co-authored-by: NJohn Law <johnlaw.po@gmail.com>

* Improve introduction part
Co-authored-by: NJohn Law <johnlaw.po@gmail.com>

project_euler/README.md

-# ProjectEuler
+# Project Euler
 
 Problems are taken from https://projecteuler.net/.
 
 Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical
 insights to solve. Project Euler is ideal for mathematicians who are learning to code.
 
-Here the efficiency of your code is also checked.
-I've tried to provide all the best possible solutions.
+The solutions will be checked by our [automated testing on Travis CI](https://travis-ci.com/github/TheAlgorithms/Python/pull_requests) with the help of [this script](https://github.com/TheAlgorithms/Python/blob/master/project_euler/validate_solutions.py). The efficiency of your code is also checked. You can view the top 10 slowest solutions on Travis CI logs and open a pull request to improve those solutions.
 
-For description of the problem statements, kindly visit https://projecteuler.net/show=all
+
+## Solution Guidelines
+
+Welcome to [TheAlgorithms/Python](https://github.com/TheAlgorithms/Python)! Before reading the solution guidelines, make sure you read the whole [Contributing Guidelines](https://github.com/TheAlgorithms/Python/blob/master/CONTRIBUTING.md) as it won't be repeated in here. If you have any doubt on the guidelines, please feel free to [state it clearly in an issue](https://github.com/TheAlgorithms/Python/issues/new) or ask the community in [Gitter](https://gitter.im/TheAlgorithms). You can use the [template](https://github.com/TheAlgorithms/Python/blob/master/project_euler/README.md#solution-template) we have provided below as your starting point but be sure to read the [Coding Style](https://github.com/TheAlgorithms/Python/blob/master/project_euler/README.md#coding-style) part first.
+
+### Coding Style
+
+* Please maintain consistency in project directory and solution file names. Keep the following points in mind:
+  * Create a new directory only for the problems which do not exist yet.
+  * If you create a new directory, please create an empty `__init__.py` file inside it as well.
+  * Please name the project directory as `problem_<problem_number>` where `problem_number` should be filled with 0s so as to occupy 3 digits. Example: `problem_001`, `problem_002`, `problem_067`, `problem_145`, and so on.
+
+* Please provide a link to the problem and other references, if used, in the module-level docstring.
+
+* All imports should come ***after*** the module-level docstring.
+
+* You can have as many helper functions as you want but there should be one main function called `solution` which should satisfy the conditions as stated below:
+  * It should contain positional argument(s) whose default value is the question input. Example: Please take a look at [problem 1](https://projecteuler.net/problem=1) where the question is to *Find the sum of all the multiples of 3 or 5 below 1000.* In this case the main solution function will be `solution(limit: int = 1000)`.
+  * When the `solution` function is called without any arguments like so: `solution()`, it should return the answer to the problem.
+
+* Every function, which includes all the helper functions, if any, and the main solution function, should have `doctest` in the function docstring along with a brief statement mentioning what the function is about.
+  * There should not be a `doctest` for testing the answer as that is done by our Travis CI build using this [script](https://github.com/TheAlgorithms/Python/blob/master/project_euler/validate_solutions.py). Keeping in mind the above example of [problem 1](https://projecteuler.net/problem=1):
+
+  ```python
+  def solution(limit: int = 1000):
+      """
+      A brief statement mentioning what the function is about.
+
+      You can have a detailed explanation about the solution method in the
+      module-level docstring.
+
+      >>> solution(1)
+      ...
+      >>> solution(16)
+      ...
+      >>> solution(100)
+      ...
+      """
+    ```
+
+### Solution Template
+
+You can use the below template as your starting point but please read the [Coding Style](https://github.com/TheAlgorithms/Python/blob/master/project_euler/README.md#coding-style) first to understand how the template works.
+
+Please change the name of the helper functions accordingly, change the parameter names with a descriptive one, replace the content within `[square brackets]` (including the brackets) with the appropriate content.
+
+```python
+"""
+Project Euler Problem [problem number]: [link to the original problem]
+
+... [Entire problem statement] ...
+
+... [Solution explanation - Optional] ...
+
+References [Optional]:
+- [Wikipedia link to the topic]
+- [Stackoverflow link]
+...
+
+"""
+import module1
+import module2
+...
+
+def helper1(arg1: [type hint], arg2: [type hint], ...) -> [Return type hint]:
+    """
+    A brief statement explaining what the function is about.
+
+    ... A more elaborate description ... [Optional]
+
+    ...
+    [Doctest]
+    ...
+
+    """
+    ...
+    # calculations
+    ...
+
+    return
+
+
+# You can have multiple helper functions but the solution function should be
+# after all the helper functions ...
+
+
+def solution(arg1: [type hint], arg2: [type hint], ...) -> [Return type hint]:
+    """
+    A brief statement mentioning what the function is about.
+
+    You can have a detailed explanation about the solution in the
+    module-level docstring.
+
+    ...
+    [Doctest as mentioned above]
+    ...
+
+    """
+
+    ...
+    # calculations
+    ...
+
+    return answer
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")
+```