Improve README

Here, we fill the core README.rst file with the following contents about OSProfiler:
1. How it works
2. Usage (With code snippets)
3. Library contents (applications to SQLAlchemy & Web)
4. Integration example (Nova)

Change-Id: I5ec0e416bc4d5d944fe2efb2965a4d03d6f75bc0
Merge "Improve README"