Add MR checklist as a version controlled file and simplify it + update contributing.md

.gitlab/merge_request_templates/default.md

@@ -0,0 +1,18 @@
+### Merge Request Checklist
+
+- [ ] [changelog.md](https://gitlab.com/bramw/baserow/-/blob/develop/changelog.md) has
+  been updated if required
+- [ ] New/updated **Premium features** are separated correctly in the premium folder
+- [ ] The latest **Chrome and Firefox** have been used to test any new frontend features
+- [ ] [Documentation](https://gitlab.com/bramw/baserow/-/tree/develop/docs) has been
+  updated
+- [ ] [Quality Standards](https://gitlab.com/bramw/baserow/-/blob/develop/CONTRIBUTING.md#quality-standards)
+  are met
+- [ ] **Performance**: tables are still fast with 100k+ rows, 100+ field tables 
+- [ ] The [redoc API pages](https://api.baserow.io/api/redoc/) have been updated for any
+  REST API changes
+- [ ] 
+  Our [custom API docs](https://gitlab.com/bramw/baserow/-/blob/develop/web-frontend/modules/database/pages/APIDocsDatabase.vue)
+  are updated for changes to endpoints accessed via api tokens
+- [ ] The UI/UX has been updated
+  following [UI Style Guide](https://baserow.io/style-guide)

CONTRIBUTING.md

@@ -38,6 +38,8 @@ repository on GitHub, but this is not the official one.
 * Documentation for every concept that can used by a plugin.
 * [changelog.md](changelog.md) should be updated with any new features.
 * The pipeline must pass.
+* Try to apply the **rule of 10s**: MRs should aim to have no more than 10 code files with more than 10 lines modified. 
+  A code file doesn't include tests/css/text/migrations/translations/configuration/ etc.
 
 ## Any contributions you make will be under the MIT Software License