"Adding Edges" section in Read The Docs

[chewselene]

More information is needed for how to add custom edges to schema.

[pmantica1]

There is currently such a section:
https://graphql-compiler.readthedocs.io/en/latest/supported_databases/sql.html#sql-edges

I do think that the current information is not the best. Specifically, I think that we should mention that before adding custom edges it is a good idea to print the schema to visualize what it looks like.