Add Makefile and include building instructions in README

Adds a Makefile to make building the project simple.

Makefile

@@ -0,0 +1,7 @@
+.PHONY: all
+.ONESHELL:
+all:
+	git submodule update --init --recursive
+	cd src/KaTeX
+	yarn
+	yarn build

README.md

@@ -23,6 +23,18 @@ A graphical editor for commutative diagrams that exports to tikz-cd.
 - Panning.
 - Support for custom macro definitions.
 
+## Building
+Make sure you have [installed yarn](https://yarnpkg.com/lang/en/docs/install/).
+
+Clone the repository. Run:
+```
+make
+```
+Open `src/index.html` in your favourite web browser.
+
+If that doesn't work, [open an issue](https://github.com/varkor/quiver/issues/new) detailing the
+problem.
+
 ## Screenshots
 The interface: