Make CONTRIBUTING.md all caps as per the Charter

According to the official Charter at
https://www.openbmc.org/wp-content/uploads/sites/62/2018/03/charter_openbmc_02142018.pdf
the file must be named literally `CONTRIBUTING`.
That is, all caps.

This has been discussed in review comments for
https://gerrit.openbmc-project.xyz/9948

This commit accompanies the corresponding changes to
the docs repository, see
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/10807

Change-Id: I320b9a4b18d4e4a741cbcb29fff8475994493ae6
Signed-off-by: Alexander Amelkin <a.amelkin@yadro.com>
diff --git a/README.md b/README.md
index 2157d7f..f41bbad 100644
--- a/README.md
+++ b/README.md
@@ -85,7 +85,7 @@
 
 ## Submitting Patches
 Support of additional hardware and software packages is always welcome.
-Please follow the [contributing guidelines](https://github.com/openbmc/docs/blob/master/contributing.md)
+Please follow the [contributing guidelines](https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md)
 when making a submission.  It is expected that contributions contain test
 cases.