Developer quick start guide

Signed-off-by: Luis Pabón <lpabon@redhat.com>
Renamed README to README.md to render in markdown

Signed-off-by: Luis Pabón <lpabon@redhat.com>
Merge pull request #2208 from lpabon/osd_dev_doc

Developer quick start guide

Reviewed-by: Sage Weil <sage@redhat.com>