Create CONTRIBUTING.md

Might require a follow-up commit to fix formatting errors as I don’t have a preview... we’ll see
Fix formattingFile_meta

ceylon/ceylon.language@e98240d changed unsafeSet to setIfAssignable.
Change german typographic apostrophes to english ones

„this“ -> “that”
Fix formattingFile_meta again

ceylon/ceylon.language@72f96c2 changed isSubTypeOf to subtypeOf.
Fix typo in generateFormattingOptions

intentMode -> indentMode
Add info about “easy”-tagged commits to CONTRIBUTING.md
WIP the rest of FormattingWriter

not tested yet because I’ll need to merge in master to make it compile,
but I think this should be good.
Merge branch 'master' into implement-7
WIP – fixes

HAHA, silly me, thinking I could be done... but seriously, now it runs,
and the only thing missing are the spaces.
Merge branch 'master' into implement-7
WIP – spaces

the first test already succeeds! Now I only have to convince the formatter to
add a line break after a multi-line comment.