docs(coding-guidelines): adding basic coding-guidelines

CONTRIBUTING.md

@@ -43,6 +43,8 @@ For general information how to develop and debug VS Code Extensions, please see
 
 The following instructions will help you run the GitLab Workflow Extension locally.
 
+Please review our [Coding guidelines](docs/coding-guidelines.md) before writing new code.
+
 #### Step - 1 : Installation Prerequisites
 
 We're assuming that you already have [Visual Studio Code](https://code.visualstudio.com/) installed along

docs/coding-guidelines.md

+# Coding guidelines
+
+We rely on `eslint` and `prettier` for code style. These guidelines contain a few rules that `eslint` and `prettier` tools don't enforce.
+
+## TypeScript
+
+- Prefer `interface` over `type` declaration when describing structures[^1].
+- If you plan to implement an interface using classes, use the `I` prefix (e.g. `Car` implements `IVehicle`)[^1].
+- Use `type` to define aliases for existing types, classes or interfaces, or to derive a type from another type. (e.g. `type PartialConfig = Partial<Config>`).
+
+[^1]: [Discussion on the MR that introduced TypeScript](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/108#note_423512996)