README: mention 'make list-defconfigs'

'make list-defconfigs' is probably one of the more useful things
to get started quickly.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

README

@@ -15,6 +15,9 @@ To build and use the buildroot stuff, do the following:
 
 You do not need to be root to build or run buildroot.  Have fun!
 
+Buildroot comes with a basic configuration for a number of boards. Run
+'make list-defconfigs' to view the list of provided configurations.
+
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 buildroot mailing list: buildroot@buildroot.org
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