Please add the corresponding seL4 repo as a submodule

[wucke13]

My current workflow for tracking microkit is:

• Find out which commit corresponds to the last release
• Browse files of the repo for that commit
• CTRL+F for seL4 in the Readme
• Copy paste the matching seL4 commit id
• Paste that into my buildscript.

From an information standpoint, mentioning the seL4 commit id in the README has a similar effect like having the git submodule file with the repo URL and commit id, just its less formal/harder to parse by tools

If the correct seL4 repo would be a submodule, my workflow could become:

• Recursively checkout the current release version of this repo

[Ivan-Velickovic]

Duplicate of #72.

[Ivan-Velickovic]

I realise that issue has been open for way too long, I'll try sort it out soon.