rust: move hgcli's README out of the way
My understanding is that it's not meant to be used in the current
form.
Differential Revision: https://phab.mercurial-scm.org/D7951
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
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273ce12ad8f1` will report some style violations on a very old commit).