Add a paragraph discouraging modification of upstream files

We have an unwritten rule in Sahara about not modifying upstream
files, even for formatting. So, now it is a written rule in the
developer guide.

Closes-Bug: #1277584
Partial-Implements: blueprint update-docs-icehouse
Change-Id: Ic018f6fce7d30a90f5a144cf8efd75cb488ab45b
Merge "Add a paragraph discouraging modification of upstream files"