Add loplugin:includeform documentation

Change-Id: I5a8c6ab0a967d9da2e1764301cd54f87f467a653
Reviewed-on: https://gerrit.libreoffice.org/61176
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

README.md

@@ -112,6 +112,8 @@ mail [Re: C[++]: Normalizing include syntax ("" vs
 The UNO API include files should consistently use double quotes, for the
 benefit of external users of this API.
 
+loplugin:includeform (compilerplugins/clang/includeform.cxx) enforces these rules.
+
 
 ## Finding out more
 

compilerplugins/clang/includeform.cxx

@@ -14,6 +14,8 @@
 #include "compat.hxx"
 #include "plugin.hxx"
 
+// Enforces the "Rules for #include directives (C/C++)" described in README.md.
+
 namespace {
 
 class IncludeForm final: public PPCallbacks, public loplugin::RewritePlugin {