Create documentation on developer guidelines

E.g.
- Apply the Pinot code style: `/config/codestyle-intellij.xml` or `/config/codestyle-eclipse.xml`
- Annotate interfaces with `InterfaceAudience`
- Use `_` as the prefix for member variables
- All upper case for constant variables
- Annotate arguments with `javax.annotation.Nullable` if it can be null; by default the argument should not be null
- SuppressWarnings annotation
- VisibleForTesting annotation

Start by reviewing the listed style references, /config/codestyle-intellij.xml and /config/codestyle-eclipse.xml, along with the guideline items in the issue. Create developer documentation covering the code style, annotations, naming conventions, and warning suppression described here. Done means each listed guideline is documented clearly for contributors.

Written by the indexing model from the issue text.