Add Database rules to readme

[Maragues]

I would like to see the database rules in Database's readme

Thanks!

[samtstern]

@Maragues are there specific rules you think we should add or do you mean a general description of DB rules?

[Maragues]

Since this is an example, I was thinking of the full JSON describing the rules, but that may be hard to maintain if the sample is still WIP.

Considering there are 3 models (User, Post and Comment), it would help seeing a real world example that manages objects that belong to other entities, authentication, flat hierarchies created to improve speed, etc.

[samtstern]

Fixed by #201, hope that helps!

Please note that in the end this is still a sample and (as I put in the updated docs) some choices were made for simplicity.

[Maragues]

Looks great, thanks!