[MRG] Locality Sensitive Hashing for approximate nearest neighbor search(GSoC)

I pointed to the class 'LSHForest' at the point when it is introduced later in this section. 
I thought providing an introduction about the usefullness of approximate neighbors at the beginning might come in handy for the users. Therefore I incluede a general introduction about LSH for approximate neighbors.

I'm not sure what needs to be changed in the description. Can you point out some places where it may look inappropriate or redundant for this document, so that I can edit it accordingly.