doc: CONTRIBUTE: tell people reviewing *is* contributing

We are "a bit" struggling with reviewing things and it's most of the
time maintainers or long time contributors reviewing patches on the
mailing list.

Hint that reviewing is also contributing to the project and that even if
you don't feel you're an expert, your review is still welcome and can
help us catch bugs before they are merged.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Tom Rini <trini@konsulko.com>

doc/CONTRIBUTE.rst

@@ -38,6 +38,15 @@ script. Please don't send patches as attachments, and ensure corporate mail
 systems don't reformat patches, append disclaimers or other unnecessary notes.
 The b4 tool automates a number of components mentioned above.
 
+Code is not the only thing you can contribute to the project. As most
+open-source projects, the U-Boot project suffers from a lack of reviewers.
+Consider spending some time reading patches on `the mailing list archive
+<https://lists.denx.de/pipermail/u-boot/>`_ and providing feedback to
+contributors when something could be improved or if you have questions. Contrary
+to what's most often believed, you do not need to be an expert to review patches
+and the project will benefit from people with different skillsets and experience
+looking at the same patches and each catch different bugs.
+
 Patch Series
 ------------