Add CONTRIBUTE.md and docker-compose file with databases setup

Author: arielj

CONTRIBUTE.md

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+# Guidelines for contributing
+
+## 1. Fork & Clone
+
+Since you probably don't have rights to the main repo, you should Fork it (big
+button up top). After that, clone your fork locally and optionally add an
+upstream:
+
+    git remote add upstream git@github.com:DatabaseCleaner/database_cleaner-active_record.git
+
+## 2. Make sure the tests run fine
+
+The gem uses Appraisal to configure different Gemfiles to test different Rails versions.
+
+- You can run all the databases through docker if needed with `docker compose up` (you can also have them running on your system, just comment out the ones you don't need from the `docker-compose.yml` file)
+- Copy `spec/support/sample.config.yml` to `spec/support/config.yml` and edit it
+- `BUNDLE_GEMFILE=gemfiles/rails_6.1.gemfile bundle install` (change `6.1` with any version from the `gemfiles` directory)
+- `BUNDLE_GEMFILE=gemfiles/rails_6.1.gemfile bundle exec rake`
+
+Note that if you don't have all the supported databases installed and running,
+some tests will fail.
+
+> Note that you can check the `.github/workflows/ci.yml` file for different combinations of Ruby and Rails that are expected to work
+
+## 3. Prepare your contribution
+
+This is all up to you but a few points should be kept in mind:
+
+- Please write tests for your contribution
+- Make sure that previous tests still pass
+- Push it to a branch of your fork
+- Submit a pull request

docker-compose.yml

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+services:
+  postgres:
+    image: postgres:16 # specify the version needed for a given app
+    environment:
+      - POSTGRES_PASSWORD=postgres # this is required
+    ports:
+      - "127.0.0.1:5432:5432" # so we can use `localhost` as the host
+  mysql:
+    image: mysql:9.3
+    environment:
+      - MYSQL_ROOT_PASSWORD=mysql
+    ports:
+      - "127.0.0.1:3306:3306"
+  redis:
+    image: redis:6.2-alpine
+    restart: always
+    ports:
+      - "127.0.0.1:6379:6379"
+    command: redis-server --save 20 1 --loglevel warning