pacific: doc/dev: add context note to dev guide config

[zdover23]

This PR adds a note directing first-time cloners of
their Ceph git forks to make sure to cd into the ceph/
directory before trying to run the "git config" commands.

Signed-off-by: Zac Dover zac.dover@gmail.com
(cherry picked from commit 3e6bcd7)

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