Added an Aggregation section to the tips documentation page.

[somada141]

Added an Aggregation section to tips.rst to show how to retrieve the session out of info and perform an aggregation query wrapping the results in a custom type and exposing it to the schema.

[coveralls]

Coverage remained the same at 91.854% when pulling 28ae4ff on somada141:feature/doc-tips-aggregation into 33d5b74 on graphql-python:master.

[coveralls]

Coverage remained the same at 91.854% when pulling 28ae4ff on somada141:feature/doc-tips-aggregation into 33d5b74 on graphql-python:master.

[coveralls]

Coverage remained the same at 91.854% when pulling 28ae4ff on somada141:feature/doc-tips-aggregation into 33d5b74 on graphql-python:master.

[Nabellaleen]

@somada141 Nice example, but is it really related to graphene ?

The "tips" is "how to retrieve easily the sqlalchemy session from graphene context", but all the "what you can do with the query" is more sqlalchemy stuff than graphene stuff, no ?
So maybe most of the code you wrote should go as example in the appropriated directory instead of in the tips ?

(we can merge that this way and create a fully functional aggragation example later)