[wp] new instructions about global cache in the tests README

src/plugins/wp/tests/README.md

@@ -59,6 +59,11 @@ doing so would causing WP to add new cache entries to the global « qualif » ca
 from all your projects around. Consult the section « Global WP Cache Setup »
 below for details.
 
+Note that this cache is meant to be global, including for the different merge
+requests. Thus, when working on a new merge request for WP, **do not** create a
+branch (and MR) on the WP-cache repository: just push the new cache entries into
+the global cache using `make wp-qualif-push`.
+
 # Qualified Test Results
 
 To be accepted by Frama-CI, tests in « qualif » configuration must be easily
@@ -75,7 +80,7 @@ are either:
 This excludes any timeout with native Alt-Ergo, which must be turned into some
 reproducible stepout by setting `-wp-steps` and `-wp-timeout` options
 accordingly. Please choose a step limit that makes large enough to ensure the
-goal is not provable, but small enough to make alt-ergo decide quickly.
+goal is provable, but small enough to make alt-ergo decide quickly.
 
 # Global WP Cache Setup (for wp-qualif)