Update README for opam

It is easy to build under opam, give explicity instructions for this.

README.org

@@ -40,3 +40,20 @@ Unfortunately, several libraries pull in camomile, which has a
 dependency on a fairly large runtime library.  TODO: Figure out how to
 point directly at this library so that this can be used on a rescue
 image.
+
+* Opam
+
+  Things are easier to install with opam.
+
+  - install opam
+
+  - opam init
+    follow the instructions
+
+  - opam install ocamlfind
+
+  - opam install ounit ocamlnet pxp sqlite3-ocaml camlzip batteries \
+       cryptokit
+
+    (note that in some instances, opam will build things multiple
+    times if you do the install steps separately)