Complete the table of methods availability

[agarie]

I wrote a table to track the methods available for each distribution, at least until most of them are implemented. It is in the README.

The problem is that it is incomplete right now. I marked most methods as unavailable, but we need to check what is available and what isn't. Also, there should be some code to know if a method is available in pure Ruby or by using the GSL/Java bindings. Not sure if that is necessary, tho.

Related to #12 and #19.

[envp]

#30 fixes this. Will make sure that future updates also update the README's table of methods.

Closing issue