Add a 'how to release' guide

Add a new HOW_TO_RELEASE.md file. This file will contain a guide explaining all the steps to achieve before to releasing a new version of Pyxel.

See these examples:

• https://github.com/pydata/xarray/blob/master/HOW_TO_RELEASE.md
• https://github.com/dask/dask/blob/master/docs/release-procedure.md

Things to add:

• a new HOW_TO_RELEASE.md file.
• add link(s) in the documentation about this release guide.