Describe the repository add procedure without any GitHub keys

Author: dsheets

README.md

@@ -0,0 +1,7 @@
+If you are adding this repository to opam without access to a secret
+key which has its public key registered with GitHub, you should use
+GitHub's public HTTPS git access like this:
+
+```
+$ opam remote add ocamllabs -k git https://github.com/ocamllabs/opam-repo-dev
+```