Add LICENSE for CommonMark spec.

Already clarify the wording of the license description in general.

README.md

@@ -41,13 +41,20 @@ The source for the documentation is in the
 All source files use the BSD 2-clause license (see [LICENSE](LICENSE)), except
 for the following:
 
--   Files in [pkg/rpc](pkg/rpc) are derived from the Go source, which uses the
-    BSD 3-clause license. See [pkg/rpc/LICENSE](pkg/rpc/LICENSE).
+-   Files in [pkg/rpc](pkg/rpc) are released under the BSD 3-clause license,
+    since they are copied from [Go's source code](https://github.com/golang/go).
+    See [pkg/rpc/LICENSE](pkg/rpc/LICENSE).
 
--   Some files in [pkg/persistent](pkg/persistent) and its subdirectories are
-    partially derived from the Clojure source code, which uses EPL 1.0. See
+-   Files in [pkg/persistent](pkg/persistent) and its subdirectories are
+    released under EPL 1.0, since they are partially derived from
+    [Clojure's source code](https://github.com/clojure/clojure). See
     [pkg/persistent/LICENSE](pkg/persistent/LICENSE).
 
+-   Files in [pkg/md/spec](pkg/md/spec) are released under the Creative Commons
+    CC-BY-SA 4.0 license, since they are derived from
+    [the CommonMark spec](https://github.com/commonmark/commonmark-spec). See
+    [pkg/md/spec/LICENSE](pkg/md/spec/LICENSE).
+
 ## Building Elvish
 
 Most users do not need to build Elvish from source. Prebuilt binaries for the

pkg/md/md_test.go

@@ -24,7 +24,7 @@ type testCase struct {
 	Name     string // supplemental cases only
 }
 
-//go:embed spec.json
+//go:embed spec/spec.json
 var specJSON []byte
 
 var testCases []testCase

pkg/md/spec/LICENSE

@@ -0,0 +1,7 @@
+The spec tests (spec.json) are derived from the CommonMark spec (spec.txt),
+which are
+
+Copyright (C) 2014-16 John MacFarlane
+
+Released under the Creative Commons CC-BY-SA 4.0 license:
+<http://creativecommons.org/licenses/by-sa/4.0/>.