Remove redundant parts of the README

These aren't specific to this repo, and are covered more generally
in the Wiki [1]. Note that:

- The claim about needing multiple Python versions is not true.
- The NPM instructions should be covered by the "make bootstrap".
- The version of Node/NPM is covered by installing the latest one.

[1]: https://github.com/alphagov/notifications-manuals/wiki/Getting-Started

Makefile

@@ -25,7 +25,7 @@ VIRTUALENV_ROOT := $(shell [ -z $$VIRTUAL_ENV ] && echo $$(pwd)/venv || echo $$V
 .PHONY: bootstrap
 bootstrap: generate-version-file
 	pip3 install -r requirements_for_test.txt
-	npm install && npm run build
+	npm install && npm rebuild node-sass npm run build
 
 .PHONY: run-flask
 run-flask: