Add "main" entry to package.json.

Give "main" a valid default value so methods like require.resolve() can
return a proper file location. Since fluent-themes doesn't have a single
"master" file, and since "main" can't refer to a folder here, point it
at a stub README file in the themes folder.

themes/README.md

@@ -0,0 +1,13 @@
+/themes/
+========
+FluentCV themes live here, one per folder.
+
+- [`minimist`][min]: A no-frills theme.
+- [`modern`][mod]: A middle of the road theme with a modern look 'n feel.
+- [`hello-world`][hw]: A simple-as-possible example theme.
+- [`compact`][com]: A compact theme.
+
+[min]: minimist
+[mod]: modern
+[hw]: hello-world
+[com]: compact