Add comments to the Makefile

Also add a Makefile script for pushing the quiver application to GitHub pages.

Makefile

@@ -1,7 +1,23 @@
-.PHONY: all
+.PHONY: all gh-pages
+
+# Ensure `cd` works properly by forcing everything to be executed in a single shell.
 .ONESHELL:
+
+# Build KaTeX.
 all:
 	git submodule update --init --recursive
 	cd src/KaTeX
 	yarn
 	yarn build
+
+# Update the quiver GitHub Pages application.
+gh-pages:
+	git subtree push --prefix src origin pre-gh-pages
+	git worktree add ../quiver-worktree pre-gh-pages
+	cd ../quiver-worktree
+	git pull
+	git checkout gh-pages
+	git rebase pre-gh-pages
+	git push --force
+	cd ../quiver
+	git worktree remove ../quiver-worktree

README.md

@@ -24,7 +24,8 @@ A graphical editor for commutative diagrams that exports to tikz-cd.
 - Support for custom macro definitions.
 
 ## Building
-Make sure you have [installed yarn](https://yarnpkg.com/lang/en/docs/install/).
+Make sure you have [installed yarn](https://yarnpkg.com/lang/en/docs/install/) and have a version of
+make that supports `.ONESHELL` (e.g. GNU Make 3.82).
 
 Clone the repository. Run:
 ```