[gnome-desktop] README: Mention bwrap non-optional requirement

[Bastien Nocera <hadess src gnome org>]

commit c244f43b7310bff36365a63ee840955bd94ee378
Author: Bastien Nocera <hadess hadess net>
Date:   Tue Dec 11 09:06:55 2018 +0100

    README: Mention bwrap non-optional requirement
    
    Closes: #82

 README | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/README b/README
index 7bf0dfed..48d63ddf 100644
--- a/README
+++ b/README
@@ -27,6 +27,9 @@ See the file 'INSTALL'. If you are not using a released version of
 gnome-desktop (for example, if you checked out the code from git), you
 first need to run './autogen.sh'.
 
+Bubblewrap, installed as the bwrap binary, is a non-optional dependency
+on platforms where it is supported and thumbnailing will silently fail
+when it is not installed at runtime.
 
 How to report bugs
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