yarn doc (#55)

* define "yarn doc"

* CI: verify docs up-to-date

* rebuild docs

* let verify steps fail verbosely

.circleci/config.yml

@@ -51,6 +51,21 @@ jobs:
           name: Run Jest
           command: node_modules/.bin/jest spec
 
+  verify_docs:
+    docker:
+      - image: circleci/node:10.15.0
+    steps:
+      - checkout
+      - restore_cache:
+          name: Restore node_modules from cache
+          key: yarn-dependencies-v1-{{ checksum "package.json" }}
+      - run:
+          name: Build docs
+          command: yarn doc
+      - run:
+          name: Verify docs up-to-date
+          command: git diff --exit-code ./docs
+
   verify_build:
     docker:
       - image: circleci/node:10.15.0
@@ -64,7 +79,7 @@ jobs:
           command: yarn build
       - run:
           name: Verify build up-to-date
-          command: git diff --quiet --exit-code ./dist
+          command: git diff --exit-code ./dist
 
 workflows:
   version: 2
@@ -78,6 +93,9 @@ workflows:
       - jest:
           requires:
             - yarn_install
+      - verify_docs:
+          requires:
+            - yarn_install
       - verify_build:
           requires:
             - yarn_install