opam: add doc field

Author: samoht

craml.opam

@@ -1,10 +1,11 @@
 opam-version: "1.2"
-maintainer: "Thomas Gazagnaire <[email protected]>"
-authors: ["Thomas Gazagnaire <[email protected]"]
-homepage: "https://github.com/realworldocaml/craml"
-license: "ISC"
-dev-repo: "https://github.com/realworldocaml/craml.git"
-bug-reports: "https://github.com/realworldocaml/craml/issues"
+maintainer:   "Thomas Gazagnaire <[email protected]>"
+authors:      ["Thomas Gazagnaire <[email protected]"]
+homepage:     "https://github.com/realworldocaml/craml"
+license:      "ISC"
+dev-repo:     "https://github.com/realworldocaml/craml.git"
+bug-reports:  "https://github.com/realworldocaml/craml/issues"
+doc:          "https://realworldocaml.github.io/craml/"
 
 build: [
  ["jbuilder" "subst" "-p" name] {pinned}