docs: contributing docs and github templates (#1496)

.github/ISSUE_TEMPLATE.md

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+<!--
+ATTENTION! WE WILL HAVE TO CLOSE THIS ISSUE if you don't provide the needed information.
+Please read https://github.com/akveo/ngx-admin/blob/master/CONTRIBUTING.md before opening an issue.
+-->
+
+### Issue type
+
+**I'm submitting a ...**  (check one with "x")
+
+* [ ] bug report
+* [ ] feature request
+* [ ] question about the decisions made in the repository
+
+### Issue description
+
+**Current behavior:**
+<!-- Describe how the bug manifests. -->
+
+**Expected behavior:**
+<!-- Describe what the behavior would be without the bug. -->
+
+**Steps to reproduce:**
+<!--  Please explain the steps required to duplicate the issue, especially if you are able to provide a sample application. -->
+
+**Related code:**
+<!-- 
+If you are able to illustrate the bug or feature request with an example, please provide a sample application via one of the following means:
+
+A sample application via GitHub
+
+StackBlitz (https://stackblitz.com)
+
+Plunker (http://plnkr.co/edit/cpeRJs?p=preview)
+
+-->
+
+```
+insert short code snippets here
+```
+
+### Other information:
+
+**npm, node, OS, Browser**
+```
+<!--
+Node, npm: `node --version` and `npm --version`
+OS: Windows (7/8/10). Linux (incl. distribution). macOS (El Capitan? Sierra?)
+Browser: Chrome/Safari/Firefox/etc?
+-->
+```
+
+**Angular, Nebular**
+```
+<!--
+Check your `package-lock.json` or locate a `package.json` in the `node_modules` folder.
+-->
+```