Create contributing guide

Add a CONTRIBUTING.md guide explaining how to build, test and setup
a development environment. Also mention the contributing guide in the
README.md.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

CONTRIBUTING.md

@@ -0,0 +1,30 @@
+Contributing to `frequenz-channels`
+===================================
+
+
+Build
+=====
+
+You can use `build` to simply build the source and binary distribution:
+
+```sh
+python -m pip install build
+python -m build
+```
+
+Local development
+=================
+
+You can use editable installs to develop the project locally (it will install
+all the dependencies too):
+
+```sh
+python -m pip install -e .
+```
+
+You can also use `nox` to run the tests and other checks:
+
+```sh
+python -m pip install nox
+nox
+```

README.md

@@ -1,3 +1,9 @@
 # Frequenz channels
 
 This repository contains channel implementations for python.
+
+## Contributing
+
+If you want to know how to build this project and contribute to it, please
+check out the [Contributing
+Guide](https://github.com/frequenz-floss/frequenz-channels/CONTRIBUTING.md).