Add more known limitations.

README/README.CXXMODULES.md

@@ -384,6 +384,13 @@ overhead which we go in details bellow.
   building pcm files with rootcling. The easiest will be to open a bug report
   to clang, however, reproducing a failure outside of rootcling is very difficult
   at the moment.
+  * Generation of RooFit dictionary hangs -- on some platforms (depending on the
+  version of libstdc++) the generation of the RooFit dictionary goes in an
+  infinite loop. We have fixed a number of such situations. Please contact us if
+  you see such behavior or disable roofit (`-Droofit=Off`).
+  * ROOT7 -- Due to number of layering violations, `-Droot7=On` does not work.
+  Please remember to explicitly switch it off when building with `-Dcxx14=On`
+  or `-Dcxx17=On`.
 
 ### Performance
 This section compares ROOT PCH technology with C++ Modules which is important but