(doc):Lack of document for upgrading ngx-admin

### Issue type
* [ ] bug report
* [x] feature request
* [ ] question about the decisions made in the repository

### Issue description
I just came across an app which is using `ngx-admin`, and I see the document is not what this app already has, and I realized it's because of the version I'm using. It's Angular version 5, and not sure about the `ngx-admin`.

**Current behavior:**
No document about how to upgrade to the newer version!

**Expected behavior:**
Document for how to update/upgrade expected.