Create a wiki page with documentation

[xflouris]

We should document the options. What I suggest we do is clearly describe the problem and how it is solved with the various methods we are implementing. Therefore, we should do the following:

• Describe the problem PTP attempts to solve (fitting branch lengths into two exponential distributions), and the motivation behind it.
• Describe the two options, single- and multi-rate solutions.
• If we are to implement GMYC then we should explain the problem/motivation as well.
• Document the options of our implementation.
• Have a clear example (a figure of a tree) with a demo execution of the program, and explain the output.
• Document the outgroup option part (how the LCA is computed, and why only two tips are required to select the LCA).

[xflouris]

started working on this