Add README text about all the language extensions needed and about using cabal-dev instead.

The cabal file keeps track of the options necessary to install the package successfully. Running cabal-dev ghci will automatically take the cabal file options into account when launching ghci. Therefore, it is probably preferable to maintaining your own .ghci file.
Suggest using cabal repl.
Suggest using cabal repl.
Merge branch 'readme_changes' of github.com:edechter/IHaskell into readme_changes
Merge 'master' changes to README.md

Clarified the difference between cabal repl and calling ghci directly and added additional language pragmas into the .ghci file.

Conflicts:
	README.md
Merge pull request #28 from edechter/readme_changes

Readme changes