all: add a section on JS/CSS formatting to README

Updates golang/go#21719

Change-Id: Ie33b439c929c466089398c4f24dc243239c48f10
Reviewed-on: https://go-review.googlesource.com/c/website/+/183879
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

.prettierrc

@@ -0,0 +1,4 @@
+{
+  "singleQuote": true,
+  "trailingComma": "es5"
+}

README.md

@@ -7,6 +7,15 @@ This repository holds the Go Website server code and content.
 The easiest way to install is to run `go get -u golang.org/x/website`. You can
 also manually git clone the repository to `$GOPATH/src/golang.org/x/website`.
 
+## JS/CSS Formatting
+
+This repository uses [prettier](https://prettier.io/) to format JS and CSS files.
+
+The version of `prettier` used is 1.18.2.
+
+It is encouraged that all JS and CSS code be run through this before submitting
+a change. However, it is not a strict requirement enforced by CI.
+
 ## Report Issues / Send Patches
 
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