Added steps to remove netlify-cms media libraries (#428)

1. I think in 99% of the case having one media library will be enough for the usecases the starter is used. By removing at least one of the media libraries the user can shave off 20 secs of build time and also the bundle size is much lower. The instructions in the `README.md` tries to explain this in a very unapinionated way
2.  Added some formatting to the commit messages, also having `[skip ci]` in the commit message will skip the build prpcess in most CIs (Netlify CI included) which i think very usefull because the user still needs to embed the images and that will also trigger a build.

Author: ZoltanVeres

README.md

@@ -80,6 +80,15 @@ CMS.registerPreviewTemplate('products', ProductPagePreview)
 CMS.registerPreviewTemplate('blog', BlogPostPreview)
 ```
 
+Note: Don't forget to also remove them from `package.json` and `yarn.lock` / `package-lock.json` using `yarn` or `npm`. During the build netlify-cms-app will bundle the media libraries as well, having them removed will save you build time.
+Example:
+```
+yarn remove netlify-cms-media-library-uploadcare
+```
+OR
+```
+yarn remove netlify-cms-media-library-cloudinary
+```
 ## Getting Started (Without Netlify)
 
 ```

static/admin/config.yml

@@ -1,6 +1,12 @@
 backend:
   name: git-gateway
   branch: master
+  commit_messages:
+    create: 'Create {{collection}} “{{slug}}”'
+    update: 'Update {{collection}} “{{slug}}”'
+    delete: 'Delete {{collection}} “{{slug}}”'
+    uploadMedia: '[skip ci] Upload “{{path}}”'
+    deleteMedia: '[skip ci] Delete “{{path}}”'
 
 media_folder: static/img
 public_folder: /img