Add preliminary developer notes.

Reviewed-by: iustinp

DEVNOTES

+Developer notes
+===============
+
+Configuring for development
+---------------------------
+
+sh autogen.sh && \
+./configure --enable-maintainer-mode \
+  --prefix=/usr/local --sysconfdir=/etc --localstatedir=/var

Makefile.am

 SUBDIRS = man lib scripts daemons doc test tools qa
-EXTRA_DIST = NEWS
+EXTRA_DIST = NEWS DEVNOTES
 
 .PHONY: ganeti
 ganeti: