document the write / read path for erasure encoding

Explains how objects are stored and used in erasure coded pools. It is
the result of discussions that occured on the ceph-devel mailing list
around june 2013. The rationale behind each change can be found in the
archive of the mailing list. For instance, the coding of the chunk
number with the object or the decision to decode using any K chunks
instead of trying to fetch the data chunks when possible because it
would allow simple concatenation when systematic codes are used.

http://tracker.ceph.com/issues/4929 refs #4929

Signed-off-by: Loic Dachary <loic@dachary.org>