Add to the documentation instructions where to add the new rules

[ernestob]

Story

Add to the documentation where the new rules/rulesets have to be added.

Background

As a new user after I installed the extension, there isn't any clue where do I have to add new rules/rulesets. It was until I came to Github and looked at all the issues that I found this reference.

Hi, you add a new rule by adding the replacerule.rules object from the README into settings.json (from the Command Palette: **Preferences: Open Settings (JSON)**). I haven't updated the instructions since the new VSCode Settings UI was introduced, so I'll look into updating it. Thanks!

Originally posted by @bhughes339 in https://github.com/bhughes339/vscode-replacerules/issues/27#issuecomment-573364247

Plan

• Update the documentation

Acceptance Criteria

• The documentation is clear on how to add rules after a new install so new users know how to use it

DOD - Definition Of Done

• Acceptance criteria met
• Deployed to the production environment
• Relevant documentation produced or updated