Add developer docs for the check interface

[nicallen]

In addition to the existing documentation on writing checks, should add more detailed documentation on the check interface that must be implemented/extended in order to create a new check.

[tromai]

I noticed that the Developer Docs for creating/extending a check has been added as part of #620. I wonder if this GitHub issue could be closed.

[behnazh-w]

Completed via #620.