add documentation on Post + Codecs

[antonkulaga]

It will be useful to have some documentation on Posts and Codecs valuable for the body part.
For instance, it is not clear to me how to send zipped file via Method.POST with hammock and how to send multipart body, where some fields are strings and some are zip files

[pepegar]

Yes, even though we have some documentation in http://pepegar.com/hammock/docs, I think we should add more to express more real use cases.