readme: update contribution guide

We move to heptapod !
2 jobs for topic/stable/contributing in 13 seconds (queued for 2 seconds)
Name Stage Failure
failed
tests Test
adding file changes
added 4934 changesets with 11470 changes to 458 files (+8 heads)
new changesets bbeef801409c:a7c01a2a3974 (12 drafts)
updating to branch default
297 files updated, 0 files merged, 0 files removed, 0 files unresolved
just before HgSha
Checking out Hg node dacbd20b2f726b5778bfe2a8c0a78fe7e845dfae as topic/stable/contributing... (skip)
abort: unknown revision 'dacbd20b2f726b5778bfe2a8c0a78fe7e845dfae'!
ERROR: Job failed: exit status 1
failed
flake8 Test
adding file changes
added 4934 changesets with 11470 changes to 458 files (+8 heads)
new changesets bbeef801409c:a7c01a2a3974 (12 drafts)
updating to branch default
297 files updated, 0 files merged, 0 files removed, 0 files unresolved
just before HgSha
Checking out Hg node dacbd20b2f726b5778bfe2a8c0a78fe7e845dfae as topic/stable/contributing... (skip)
abort: unknown revision 'dacbd20b2f726b5778bfe2a8c0a78fe7e845dfae'!
ERROR: Job failed: exit status 1