Improved developer guide and fixed typos.

- Added a class diagram of compare's metamodel overview
- Replaced " === " in code extracts by " = "
- Tried to improve some sentences for readability

Change-Id: I3bb856100d37886d8721965663ff05b629dcb8c1
Signed-off-by: Laurent Delaigue <laurent.delaigue@obeo.fr>