Add npm installation step to Makefile and update README

Author: 7h3kk1d

Makefile

@@ -9,6 +9,7 @@ all: dev
 deps:
 	opam update
 	opam install ./hazel.opam.locked --deps-only --with-test --with-doc
+	npm install
 
 change-deps:
 	opam update

README.md

@@ -17,7 +17,7 @@ can also be accessed at:
 
 ### Short version
 
-If you already have `ocaml` version 5.2.0 and at least version 2.0 of `opam`
+If you already have `npm`, `ocaml` version 5.2.0 and at least version 2.0 of `opam`
 installed, you can build Hazel by running the following commands.
 
 - `git clone [email protected]:hazelgrove/hazel.git`