Adding doc about different uses of catkin_pip

[asmodehn]

Use cases :

• Usual Python ROS package => no need of python or pip, but still works
• Pure Python in ROS => 2 steps :
  • From source : python project and use pip in devel workspace
  • Build package : work with install workspace to find and solve dependencies issues.
• Using a python project as thirdparty package. Ideally without modifying original repo.

Note : Detail ROS workflow with prereleases, and workflow from thirdparty packages, tested in travis.

[asmodehn]

Working on case 3 right now. For the sake of documentation later on :

1. Make sure the tag exist in the original source repo (called upstream compared to the release repo)
2. Create the release repo (name it <package_name>-rosrelease to be more explicit than just <package_name>-release)
3. Add the patches you need in that release repo (travis + others ? maybe package.xml... ) so bloom doesn't complain later. Strategy here is to have one patch directory per track/distro.
4. Add the repository to your travis, so the next release is tested.
5. Use bloom-release to create a thirdparty release:
   • bloom-release --rosdistro jade --track jade pyzmp
   • Version :{ask} or :{auto}
   • Release Tag ros-:{version} to have special tags for ros package release
   • Upstream Devel branch None to use default branch of the repository (CAREFUL with conflicts : This is useful for prereleases in ROS workflow)
   • Patch Directory jade-patches to have different patches per track/rosdistro
6. Wait until travis test passes to do the pull request. If a test fail you can start again the process.

[asmodehn]

• documentation on source repo (until we generate from rosrelease with link to python doc from upstream repo)
• source from upstream repo (maybe better to point to release version for the distro ?)

[asmodehn]

Looking at the documentation durig bloom release process, tag :{auto} is not available when doing third party release, because package.xml is not in upstream repo.

[asmodehn]

Related to this the thirdparty workflow could benefit from http://travis.debian.net/
We should investigate and if successful document its use.