Update kss style guide wording and README

This should explain the conventions of the MediaWiki project.
Wikimedia is a prominent user of MediaWiki, but not the only one.

Other MediaWiki installations do not share all of Wikimedia's
values and mission.  However, it is possible to express common values
that should guide this work in core.

Thus, I retained some of the existing text, as well as adding one other
point, accessibility (which we have discussed but was not mentioned
here).

Change-Id: Iafc8641c9b89138b3e4b7f6e274395279ddad43c
Merge "Update kss style guide wording and README"