Fix release script after experience from 0.7.1 release

* Also edit dist/README.md. Otherwise npm publish will overwrite that
  AFTER the dist directory has been added, causing the git checkout to fail.
  And it's the right thing to do anyway, having ALL the READMEs edited.
* Add all the modified READMEs so they get committed correctly.
* Improve two references to master which are no longer accurate.
* Check for uncommitted changes just before creating the tag.
* Encourage always specifying the next version, as discussed in
  https://github.com/Khan/KaTeX/pull/615#discussion_r97208770

release.sh

@@ -24,9 +24,12 @@ usage() {
     echo ""
     echo "Examples:"
     echo " When releasing a new point release:"
-    echo "   ./release.sh 0.6.3"
+    echo "   ./release.sh 0.6.3 0.6.4"
     echo " When releasing a new major version:"
     echo "   ./release.sh 0.7.0 0.8.0"
+    echo ""
+    echo "You may omit NEXT_VERSION in order to avoid creating a commit on"
+    echo "the branch from which the release was created.  Not recommended."
     exit $1
 }
 
@@ -114,11 +117,12 @@ rm -f package.json.bak
 
 # Update the version number in CDN URLs included in the README files,
 # and regenerate the Subresource Integrity hash for these files.
-node update-sri.js "${VERSION}" README.md contrib/*/README.md
+node update-sri.js "${VERSION}" README.md contrib/*/README.md dist/README.md
 
 # Make the commit and tag, and push them.
-git add package.json bower.json
+git add package.json bower.json README.md contrib/*/README.md dist/README.md
 git commit -n -m "v$VERSION"
+git diff --stat --exit-status # check for uncommitted changes
 git tag -a "v$VERSION" -m "v$VERSION"
 git push origin "v$VERSION"
 
@@ -126,7 +130,7 @@ git push origin "v$VERSION"
 npm publish
 
 if [ ! -z "$NEXT_VERSION" ]; then
-    # Go back to master to bump
+    # Go back to original branch to bump
     git checkout "$BRANCH"
 
     # Edit package.json and bower.json to the right version
@@ -137,8 +141,8 @@ if [ ! -z "$NEXT_VERSION" ]; then
     # development branch, too.  Most people will read docs on master.
     node update-sri.js "${VERSION}" README.md contrib/*/README.md
 
-    git add package.json bower.json
-    git commit -n -m "Bump master to v$NEXT_VERSION-pre"
+    git add package.json bower.json README.md contrib/*/README.md
+    git commit -n -m "Bump $BRANCH to v$NEXT_VERSION-pre"
     git push origin "$BRANCH"
 
     # Go back to the tag which has build/katex.tar.gz and build/katex.zip