[2145] added some comments about in-source test setup for bind10 script

src/bin/bind10/bind10.in

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+# We use this wrapper script both for production and in-source tests; in
+# the latter case B10_FROM_BUILD environment is expected to be defined.
 if test -n "${B10_FROM_BUILD}"; then
 	exec ${B10_FROM_BUILD}/src/bin/bind10/b10-init $*
 else