Move mdast types to dev dependency

Closes GH-6.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: ChristianMurphy

package.json

@@ -32,10 +32,10 @@
     "index.js"
   ],
   "dependencies": {
-    "@types/mdast": "^3.0.0",
     "unist-util-visit": "^2.0.0"
   },
   "devDependencies": {
+    "@types/mdast": "^3.0.0",
     "browserify": "^16.0.0",
     "dtslint": "^3.0.0",
     "nyc": "^15.0.0",

readme.md

@@ -20,6 +20,12 @@ Supports funky keys, like `__proto__` or `toString`.
 npm install mdast-util-definitions
 ```
 
+[npm][] with [TypeScript][] support:
+
+```sh
+npm install mdast-util-definitions @types/mdast
+```
+
 ## Use
 
 ```js
@@ -123,6 +129,8 @@ abide by its terms.
 
 [npm]: https://docs.npmjs.com/cli/install
 
+[typescript]: https://www.typescriptlang.org/
+
 [contributing]: https://github.com/syntax-tree/.github/blob/master/contributing.md
 
 [support]: https://github.com/syntax-tree/.github/blob/master/support.md