(docs): run doctoc on pre-commit (#399)

- and install it as a devDep

- doctoc is used in the README ToC and listed in comment, but wasn't
  a devDep or used in any internal scripts

package.json

@@ -116,14 +116,15 @@
     "@types/react": "^16.9.11",
     "@types/rollup-plugin-json": "^3.0.2",
     "@types/rollup-plugin-sourcemaps": "^0.4.2",
+    "doctoc": "^1.4.0",
     "husky": "^3.0.9",
     "pretty-quick": "^2.0.0",
     "ps-tree": "^1.2.0",
     "react": "^16.8.6"
   },
   "husky": {
     "hooks": {
-      "pre-commit": "pretty-quick --staged --pattern '!test/tests/lint/**' && yarn lint"
+      "pre-commit": "pretty-quick --staged --pattern '!test/tests/lint/**' && yarn lint && doctoc README.md"
     }
   },
   "prettier": {