Add wiki documentation about index.yaml

[jaugust4]

Added documentation requested in issue #152

Ready to Review and Edit

[seanlip]

Hi @jaugust4, thanks for your PR. I think the thrust of this page is a bit off. index.yaml is already set up and exists for this project, so all the stuff about "how to set up index.yaml from scratch" is irrelevant.

I think this page should explain:

• what index.yaml is and what it's used for
• what sorts of datastore queries will require updates to index.yaml, and which don't
• what developers should do in index.yaml when adding a new multi-field datastore query

Could you please update the page to match the above, and also ensure that you test the flows locally by trying out appropriate changes on the oppia/oppia repo? Please also explain, in your response to this review, how you verified the instructions.

Thanks!