erasure-code: document pool operations

A short introduction to the first time user of an erasure coded pool.
It includes a reminder of how it relates to cache tiering and links to
define new profiles with an example.

There was examples in the developer documentation but the operator
expects to find such a guide in the rados operations chapter.

http://tracker.ceph.com/issues/9970 Fixes: #9970

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #2888 from dachary/wip-9970-erasure-code-documentation

erasure-code: document pool operations

Reviewed-by: Laurent Guerby <laurent@guerby.net>