Turn ChangeLog/README into a Markdown file and expand contents of README

[Vlix]

Changing it to ChangeLog.md will make it use markdown formatting when viewing on hackage. That and crediting the PRs that added the changes would be nice for posterity and showing thanks for anyone helping out with the library.

[Vlix]

Also a good idea to mention in the README that everything in the other submodules gets exported from the main one and add the suggestion to import Network.HTTP.Types qualified when using anything from this package.

And just to explain more about the package in the README. Contents, functionality, etc.

[Vlix]

N.B. don't forget to also change the ChangeLog and README to ChangeLog.md and README.md in the http-types.cabal file.

[Vlix]

Fixed in #19, and #23 adds more documentation to README.md