REP for releasing third party packages.
Added a benefit to having a package.xml upstream

contributed by @dirk-thomas
Added need for <build_type> tag in the package.xml
Added tutorial link and examples
Taking REP number 136, marking Informational
Added reference to mailing list for citation
Added some notes about what a third party package is and why we have them
Added Consequences section

This section describes the consequences of this
recommendation as it relates to dependent packages
finding and using third party packages.
Small typo, reworded to be more clear about upstream package.xml
Merge pull request #28 from ahornung/patch-1

Small typo, reworded to be more clear about upstream package.xml
Added clarifications on the scope of this REP

There were concerns from the mailing list that
the REP's scope was not clearly defined.
Spelling and grammatical corrections
Added subsection on backwards compatibility.

Specifically to address backwards compatibility
with legacy rosbuild packages.
Style fixes.
Update rep-0136.rst

Setting REP status to Active
Merge pull request #23 from ros-infrastructure/release_third_party

REP 136 for releasing third party packages