Add .nvmrc & use NVM to manage installed Node version

We have decided to use NVM to manage installed Node versions locally
and in CI to ensure they match and produce consistent builds.

Running `nvm install` will install the Node version specified in the
`.nvmrc` file.

This is now consistent with Document Download Frontend.
See: alphagov/document-download-frontend#114

Signed-off-by: Richard Baker <richard.baker@digital.cabinet-office.gov.uk>

.nvmrc

@@ -0,0 +1 @@
+16.14.0

Makefile

@@ -25,8 +25,8 @@ VIRTUALENV_ROOT := $(shell [ -z $$VIRTUAL_ENV ] && echo $$(pwd)/venv || echo $$V
 .PHONY: bootstrap
 bootstrap: generate-version-file ## Set up everything to run the app
 	pip3 install -r requirements_for_test.txt
-	npm install
-	. environment.sh; npm run build
+	source $(HOME)/.nvm/nvm.sh && nvm install && npm ci
+	. environment.sh; source $(HOME)/.nvm/nvm.sh && npm run build
 
 .PHONY: watch-frontend
 watch-frontend:  ## Build frontend and watch for changes
@@ -56,7 +56,7 @@ generate-version-file: ## Generates the app version file
 test: ## Run tests
 	flake8 .
 	isort --check-only ./app ./tests
-	npm test
+	source $(HOME)/.nvm/nvm.sh && npm test
 	py.test -n auto --maxfail=10 tests/
 
 .PHONY: fix-imports

README.md

@@ -13,17 +13,21 @@ GOV.UK Notify admin application - https://www.notifications.service.gov.uk/
 
 At the moment we run Python 3.9 in production.
 
-### NPM packages
+### NodeJS & NPM
+
+If you don't have NodeJS on your system, install it with homebrew.
 
 ```shell
 brew install node
 ```
 
-[NPM](npmjs.org) is Node's package management tool. `n` is a tool for managing different versions of Node. The following installs `n` and uses the long term support (LTS) version of Node.
+`nvm` is a tool for managing different versions of NodeJS. Follow [the guidance on nvm's github repository](https://github.com/nvm-sh/nvm#installing-and-updating) to install it.
+
+Once installed, run the following to switch to the version of NodeJS for this project. If you don't
+have that version, it should tell you how to install it.
 
 ```shell
-npm install -g n
-n lts
+nvm use
 ```
 
 ### `environment.sh`
@@ -75,4 +79,5 @@ To run a specific JavaScript test, you'll need to copy the full command from `pa
 ## Further docs
 
 - [Working with static assets](docs/static-assets.md)
+- [JavaScript documentation](https://github.com/alphagov/notifications-manuals/wiki/JavaScript-Documentation)
 - [Updating dependencies](https://github.com/alphagov/notifications-manuals/wiki/Dependencies)