Create a Contributing page in the documentation (#190)

docs/contributing.md

@@ -0,0 +1,44 @@
+# Contributing
+
+You are welcome to contribute with **HTTPX**, read this guide carefully to
+understand how to setup your environment.
+
+## Development
+
+To start developing **HTTPX** create a **fork** of the
+[httpx repository](https://github.com/encode/httpx) on GitHub.
+
+Then clone your fork with the following command replacing `YOUR-USERNAME` with
+your GitHub username:
+
+```shell
+$ git clone https://github.com/YOUR-USERNAME/httpx
+```
+
+## Testing
+
+We use [nox](https://nox.thea.codes/en/stable/) as testing tool, so before
+testing make sure you have it installed at your system.
+
+You can install nox with:
+
+```shell
+$ python3 -m pip install --user nox
+```
+
+Or if you prefer to keep it into an isolated environment you can install it
+using [pipx](https://github.com/pipxproject/pipx):
+
+```shell
+$ pipx install nox
+```
+
+Now with nox installed you can run the tests by running:
+
+```shell
+$ nox
+```
+
+!!! warning
+    The test suite spawns a testing server at the port **8000**.
+    Make sure this isn't being used, so the tests can run properly.

mkdocs.yml

@@ -16,6 +16,7 @@ nav:
     - Async Client: 'async.md'
     - Requests Compatibility: 'compatibility.md'
     - Developer Interface: 'api.md'
+    - Contributing: 'contributing.md'
 
 markdown_extensions:
   - admonition