Add note on how to generate proto files to docs

Summary:
Add a note on how to generate .proto files properly to the docs.

Raised in https://phabricator.ashoat.com/D2790#71108

Test Plan: None

Reviewers: palys-swm, geekbrother, atul, ashoat

Reviewed By: palys-swm, geekbrother, ashoat

Subscribers: anand, benschac, ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D2844