Add basic documentation related to development

[psibi]

It would be nice if things related to development also gets documented somewhere.

Example: If you change the build system, all you have to do is stack build eta:eta-build to build the new eta-build instead of doing cleaninstall.sh which I was doing earlier.

[rahulmutt]

Posting here for when we create the dev section in the docs:

Grepping through Eta code base with regexes:

For function definitions: ^[function-name]
For data declarations: ^data.*[type-name]
For newtype/type synonym declarations: type\ [type-name]
For record field declarations: [field-name].*::