Create transitioning-contractor-code-to-innersource-model

[claredillon]

Added new pattern as discussed in https://github.com/InnerSourceCommons/InnerSourcePatterns/issues/375

[claredillon]

@zkoppert - can you take a look at this please and see if you would add any details. It would be great if we could add some explicit steps to transition if we can...

[spier]

Nice job @claredillon! I added the .md extension to the file.

This also results in our markdown syntax linter to check this file, which is why you should have received some email notifications with syntax warnings. I tried to fix some of them but not sure if I found all.

[zkoppert]

@claredillon It looks like some of @spier's comments are directed towards your part of the content. Just dropping your name here as a reminder to address those comments.

[spier]

@claredillon how shall we proceed on this PR?