CONTRIBUTING: make S-o-b mandatory but pseudonym ok

This was prompted by a recent discussion on #dri-devel IRC channel,
where the use of pseudonyms to maintain anonymity was said to be a
normal and accepted practice. See:
https://people.freedesktop.org/~cbrill/dri-log/index.php?channel=dri-devel&highlight_names=&date=2021-02-09
and look for the discussion around pq and Lyude.
Until then, I was hesitant to accept Signed-off-by's with names that
looked very much not a real name.

Clarify our documentation that pseudonyms are ok.

Note, that is not what the Linux kernel documentation says today in
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst
which says that pseudonyms are not ok. According to the discussion, this
should probably be fixed in the kernel too.

Since we are now ok with pseudonyms in Signed-off-by, there is no reason
left to accept any patches without a Signed-off-by. This clarifies our
policy and takes the burden of case-by-case consideration away from
maintainers.

The wording about needing to use a personal email address is my
addition. The intention is to ensure a globally unique handle for a
person while that person remains anonymous, so that the Signed-off-by
cannot be mistaken for someone elses.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

CONTRIBUTING.md

@@ -96,17 +96,20 @@ cope with the way git log presents them.
 See [notes on commit messages] for a recommended reading on writing commit
 messages.
 
-Your patches should also include a Signed-off-by line with your name and
+Your patches must also include a Signed-off-by line with your name
-email address which indicates that you agree to the
+(or pseudonym) and email address which indicates that you agree to the
 [Developer's Certificate of Origin 1.1](DCO-1.1.txt).
 If you're not the patch's original author, you should
-also gather S-o-b's by them (and/or whomever gave the patch to you.) The
+also gather S-o-b's from them (and/or whomever gave the patch to you) in
+addition to your own S-o-b. The
 significance of this is that it certifies that you created the patch,
 that it was created under an appropriate open source license, or
 provided to you under those terms.  This lets us indicate a chain of
 responsibility for the copyright status of the code.
 
-We won't reject patches that lack S-o-b, but it is strongly recommended.
+**Agreeing to DCO 1.1 is mandatory.** Patches without a Signed-off-by cannot
+be accepted, but using a pseudonym is fine as long as the email address is
+yours personally.
 
 When you re-send patches, revised or not, it would be very good to document the
 changes compared to the previous revision in the commit message and/or the