Add README-EXTERNALS with instructions for fixing checkouts after the

impending externals merge.

README-EXTERNALS

@@ -0,0 +1,26 @@
+If you are reading this file you have probably just seen an error like
+this after updating:
+
+svn: URL 'svn://svn.insecure.org/nping' of existing directory 'nping'
+does not match expected URL 'svn://svn.insecure.org/nmap/nping'
+
+This is caused by the replacement of SVN externals.
+
+Here's what you need to do. First, save any local changes you might have
+in the nping, nsock, nbase, ncat, and zenmap directories. (For example
+by running "cd nping; svn diff > ../nping.diff".) If you don't have any
+local changes you can skip this step.
+
+Then run these commands:
+
+rm -rf nping/ nsock/ nbase/ ncat/ zenmap/
+svn update
+svn cleanup
+
+If all else fails, you can just delete your whole working directory and
+check out anew:
+
+svn co --username guest --password "" svn://svn.insecure.org/nmap
+
+There may be further discussion in the mailing list thread at
+http://seclists.org/nmap-dev/2011/q4/303.