Created Contribution guide (markdown)

Contribution-guide.md

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+**Don't use Docker to do development**. It's a quick way to get Mastodon running in production, it's **really really inconvenient for development**. Normally in Rails development environment you get hot reloading of backend code and on-the-fly compilation of assets like JS and CSS, but you lose those benefits by compiling a Docker image. If you want to contribute to Mastodon, it is worth it to simply set up a proper development environment.
+
+In fact, all you need is described in the production guide, **with the following exceptions**. You **don't** need:
+
+- Nginx
+- SystemD
+- An `.env.production` file. If you need to set any environment variables, you can use an `.env` file
+- To prefix any commands with `RAILS_ENV=production` since the default environment is "development" anyway
+- Any cronjobs
+
+The command to install project dependencies does not require any flags, i.e. simply
+
+    bundle install
+
+By default the development environment wants to connect to a `mastodon_development` database on localhost using your user/ident to login to Postgres (i.e. not a md5 password)
+
+You can run Mastodon with:
+
+    rails s
+
+And open `http://localhost:3000` in your browser. Background jobs run inline (aka synchronously) in the development environment, so you don't need to run a Sidekiq process.
+
+You can run tests with:
+
+    rspec
+
+You can check localization status with:
+
+    i18n-tasks health
+
+You can check code quality with:
+
+    rubocop