Document that we don't want pushed commits to be rebased and give a rationale for many small MR's policy.
Document that we don't want pushed commits to be rebased and give a rationale for many small MR's policy.

8thcolor/eurucamp2014-htdsadr,8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr,8thcolor/rubyconfau2015-sadr,8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr