REAMDE: Mention that gitlab's 'download' button doesn't work.

This is because the `git archive` command doesn't preserve submodules.

Note that currently there is no support for disabling the 'download' button
in gitlab (https://gitlab.com/gitlab-org/gitlab/-/issues/17032).

Fixes issue #1158.

README

@@ -16,7 +16,9 @@ Read the files `docs/INSTALL*`  for installation instructions; see the
 file `docs/LICENSE.TXT` for the available licenses.
 
 For using FreeType's git repository  instead of a distribution bundle,
-please read file `README.git`.
+please read file  `README.git`.  Note that you have  to actually clone
+the repository; using a snapshot will  not work (in other words, don't
+use gitlab's 'Download' button).
 
 The FreeType 2 API reference is located in directory `docs/reference`;
 use the file  `index.html` as the top entry point.   [Please note that