Add a basic CONTRIBUTING.md

CONTRIBUTING.md

+Contributing to Sky Engine
+==========================
+
+Getting the code
+----------------
+
+To get the code:
+
+ 1. Fork https://github.com/domokit/sky_engine into your own GitHub account.
+ 2. [Download depot_tools](http://www.chromium.org/developers/how-tos/install-depot-tools)
+    and make sure it is in your path.
+ 3. Create a `.gclient` file in an empty directory with the following contents:
+
+```
+solutions = [
+  {
+    "managed": False,
+    "name": "src",
+    "url": "git@github.com:<your_name_here>/sky_engine.git",
+    "custom_deps": {},
+    "deps_file": "DEPS",
+    "safesync_url": "",
+  },
+]
+target_os = ["android"]
+```
+
+ 4. `svn ls https://dart.googlecode.com/` and accept the certificate permanently.
+ 5. `gclient sync`
+ 6. `cd src`
+ 7. `git remote add upstream git@github.com:domokit/sky_engine.git`
+ 8. `./build/install-build-deps.sh`
+
+Building the code
+-----------------
+
+ 1. `./mojo/tools/mojob gn`
+ 2. `ninja -C out/Debug`
+
+Contributing code
+-----------------
+
+The Sky engine repository gladly accepts contributions via GitHub pull requests.