fix and add readme CI badges

[compwron]

https://github.com/rubyforgood/Flaredown/blob/master/README.md

[![rspec](https://github.com/rubyforgood/Flaredown/actions/workflows/rspec.yml/badge.svg)](https://github.com/rubyforgood/Flaredown/actions/workflows/rspec.yml)
[![frontend](https://github.com/rubyforgood/Flaredown/actions/workflows/frontend.yml/badge.svg)](https://github.com/rubyforgood/Flaredown/actions/workflows/frontend.yml)
[![ERB lint](https://github.com/rubyforgood/Flaredown/actions/workflows/erb_lint.yml/badge.svg)](https://github.com/rubyforgood/Flaredown/actions/workflows/erb_lint.yml)
[![standardrb lint](https://github.com/rubyforgood/Flaredown/actions/workflows/ruby_lint.yml/badge.svg)](https://github.com/rubyforgood/Flaredown/actions/workflows/ruby_lint.yml)