README: Say that files must be listed in WHENCE, and how to check it

- Say that new files must, not 'should', be listed in WHENCE
- State the expected form of a reference to a separate licence file
- Mention 'make check'
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

README

@@ -16,16 +16,19 @@ diff or preferably a git pull request to:
       linux-firmware@kernel.org
 and also cc: to related mailing lists.
 
-Your commit should include an update to the WHENCE file clearly
-identifying the license under which the firmware is available, and
+If your commit adds new firmware, it must update the WHENCE file to
+clearly state the license under which the firmware is available, and
 that it is redistributable. Being redistributable includes ensuring
 the firmware license provided includes an implicit or explicit
 patent grant to end users to ensure full functionality of device
 operation with the firmware. If the license is long and involved, it's
 permitted to include it in a separate file and refer to it from the
-WHENCE file.
+WHENCE file ('See LICENSE.foo for details.').
 And if it were possible, a changelog of the firmware itself.
 
+Run 'make check' to check that WHENCE is consistent with the
+repository contents.
+
 Ideally, your commit should contain a Signed-Off-By: from someone
 authoritative on the licensing of the firmware in question (i.e. from
 within the company that owns the code).