Add Arch AUR install info in README.md

<!-- Try not to make duplicates. Do a quick search before posting LANGUAGES NOTE: Please check https://developers.google.com/assistant/sdk/reference/rpc/languages to check if the language you are adding is supported. --> ### Description What does this PR add or fix? Add the details here Add install steps for Arch AUR ### Resolved / Related Issues Issues resolved or addressed by this PR. - Closes #478 - Addresses #97 ### Type of Change - [ ] Feature - [ ] Bug Fix - [x] Documentation - [ ] Localization ### Validation Checklist Check all that applies - [ ] Built and ran the app - [ ] Tested my changes _(to make sure your changes don't break something, make sure to test the code)_ - [ ] Formatted code using Prettier/ESLint - [ ] Added comments/documentation in relevant places _([JSDocs](https://jsdoc.app/) for function/class body and `//` comments for other parts)_ ### Screenshots (optional) Add screenshots here.