Add a developer guide doc

Adding a new developer guide document generally takes three steps:

1. Write an rst file for doc

Write a new rst file for developer guide document, which should be placed correspondingly in valence/doc/source.

2. Check TOC tree for docs

We should check TOC tree ref for your newly added rst file, ensure that your rst file can be generated according to valence/doc/source/index.rst.

3. Generate docs

Regenerate all the docs including your newly added rst file:

$ tox -e docs

Or, directly run sphinx in valence/doc:

$ python setup.py build_sphinx