Add to README Documentation Building

[ramanxg]

Following the README instructions, the site won't actually load, because the documentation isn't built yet. The commands to build are

Note in instructions, that this isn't necessary, unless viewing the documentation.

• pip install -r docs_requirements.txt
• mkdocs build -d docs-site

Include link to https://api.peterportal.org/Contributing/documenting/, as it is right now, but the wording needs to be changed.