reworked examples folder and fixed Readme

[StrikerRUS]

This PR moves RGF examples into separate subfolder RGF and renames fast_rgf to FastRGF.

Also Troubleshooting section is reworked (the main change - #13 is no more actual since we are checking input X and understandable exception is raised in such cases).

[fukatani]

LGTM!
And we can change description to
The wrapper of machine learning algorithm Regularized Greedy Forest and FastRGF.
in readme

[StrikerRUS]

I believe that the algorithm is RGF, but one of the implementation is named FastRGF.
I think (will create a PR) that the note about supporting two backends shoud be placed in Features section of Readme.