[tests] Update WP tests README

src/plugins/wp/tests/README.md

+# Running qualif tests
+
+- Be sure that you have installed the appropriate versions of
+  alt-ergo and coq _before_ having compiled Frama-C.
+- use `make wp-qualif` in the toplevel Frama-C directory
+
 # Test Suites
 
 Here is a short description of the WP test suites: