README: add generic notes about GENERIC and NOTES

Inform new users what GENERIC and NOTES are. These are useful for people
perusing the tree without a great deal of specific fbsd knowledge.
See discussion of r334073 for further motivation.

Requested by:	jhb

Author: grimreaper

README

@@ -60,7 +60,9 @@ stand		Boot loader sources.
 
 sys		Kernel sources.
 
-sys/<arch>/conf Kernel configuration file
+sys/<arch>/conf Kernel configuration files. GENERIC is the configuration
+		used in release builds. NOTES contains documentation of
+		all possible entries.
 
 tests		Regression tests which can be run by Kyua.  See tests/README
 		for additional information.

README.md

@@ -62,7 +62,9 @@ stand		Boot loader sources.
 
 sys		Kernel sources.
 
-sys/<arch>/conf Kernel configuration file
+sys/<arch>/conf Kernel configuration files. GENERIC is the configuration
+		used in release builds. NOTES contains documentation of
+		all possible entries.
 
 tests		Regression tests which can be run by Kyua.  See tests/README
 		for additional information.