Fill in the slapproxy README, states about difference from Master

slapos/README.proxy.txt

 proxy
 =====
+
+Implement minimalist SlapOS Master server without any security, designed to work
+only from localhost with one SlapOS Node (a.k.a Computer).
+
+It implements (or should implement) the SLAP API, as currently implemented in
+the SlapOS Master (see slaptool.py in Master).
+
+The only behavioral difference from the SlapOS Master is:
+
+When the proxy doesn't find any free partition (and/or in case of slave
+instance, any compatible master instance), it will throw a NotFoundError (404).