Collect and add alternatives to readme

[xiegeo]

There are two classes of alternatives. Those that do the same job slightly differently (list pros and cons), and those that do a different job (you should use them if). This is intended to help visitors choose a library to use and should belong in the readme file.

[xiegeo]

A wiki page have been created for this: https://github.com/xiegeo/must/wiki/Alternatives

Todo:

• add link to wiki page from readme.