README: add instruction to run Vale on a subset

make stylecheck runs Vale on the whole documentation which can be long
and reports a lot of errors/warnings. We can run Vale on a subset using
the VALEDOCS variable, so update documentation to highlight it.

(From yocto-docs rev: 262237f72534c983e178231cb6839ed69709c443)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Tested-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/README

@@ -142,6 +142,14 @@ To run Vale:
 
  $ make stylecheck
 
+Style checking the whole documentation might take some time and generate a
+lot of warnings/errors, thus one can run Vale on a subset of files or
+directories:
+
+  $ make stylecheck VALEDOCS=<file>
+  $ make stylecheck VALEDOCS="<file1> <file2>"
+  $ make stylecheck VALEDOCS=<dir>
+
 Link checking the Yocto Project documentation
 =============================================