Document coding conventions

Review this:

https://incatools.github.io/ontology-access-kit/best-practice.html

Maybe doesn't need to be in main docs but we should decide on other best practice such as

- shifting from start of list; see #148

Start by reviewing the linked best-practice page and issue #148. Determine whether coding conventions belong in the main documentation or elsewhere, including the proposed practice of shifting from the start of a list. Done means the documentation location and relevant conventions have been decided and recorded.

Written by the indexing model from the issue text.