manual: remove contributor list and add copyright

manual: remove contributor list and add copyright

As discussed on the mailing list [1], remove the limited explicit list of
contributors in favor of the general mention of 'The Buildroot developers'.
Add a copyright statement.
Move the generation info to the front.

[1] http://lists.busybox.net/pipermail/buildroot/2014-May/097589.html

[Peter: remove trailing +, minor rewording]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>