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:

README.md

@@ -11,20 +11,10 @@ GOV.UK Notify admin application - https://www.notifications.service.gov.uk/
 
 ## Setting up
 
-### Install Homebrew
-
-Install [Homebrew](https://brew.sh), a package manager for OSX:
-
-```shell
-/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
-```
-
 ### Make sure you're using correct language versions
 
 Languages needed
 - Python 3.6.x
-- [Node](https://nodejs.org/) 10.15.3 or greater
-- [npm](https://www.npmjs.com/) 6.4.1 or greater
 
 Need to install node? Run:
 
@@ -32,16 +22,6 @@ Need to install node? Run:
 brew install node
 ```
 
-#### `pyenv` For Python version management
-
-[pyenv](https://github.com/pyenv/pyenv) is a program to manage and swap between different versions of Python. To install:
-
-```shell
-brew install pyenv
-```
-
-And then follow the further installation instructions in https://github.com/pyenv/pyenv#installation to configure it.
-
 #### `n` For Node version management
 
 [NPM](npmjs.org) is Node's package management tool. `n` is a tool for managing
@@ -53,13 +33,6 @@ npm install -g n
 n lts
 ```
 
-### Install NPM dependencies
-
-```shell
-npm install
-npm rebuild node-sass
-```
-
 ### Install Python dependencies
 
 Install dependencies and build the frontend assets:
@@ -68,12 +41,6 @@ Install dependencies and build the frontend assets:
 make bootstrap
 ```
 
-**Note:** You may need versions of both Python 3 and Python 2 accessible to build the python dependencies. `pyenv` is great for that, and making both Python versions accessible can be done like so:
-
-```shell
-pyenv global 3.6.3 2.7.15
-```
-
 ### Create a local `environment.sh` file
 
 In the root directory of the application, run: