Merge pull request #4907 from CocoaPods/mr-issue-template

[ISSUE_TEMPLATE] Initial proposal

.github/ISSUE_TEMPLATE.md

+> ℹ Please fill out this template when filing an issue.
+> All lines beginning with a quotation mark (`>`),
+> instruct you with what info we expect.  
+> Please remove this paragraph.
+
+* [ ] I've read and understood the [*CONTRIBUTING guidelines and have done my best effort to follow](https://github.com/CocoaPods/CocoaPods/blob/master/CONTRIBUTING.md).
+
+### Report
+
+#### What did you do?
+
+> ℹ Please replace this with what you did.  
+> e.g. Run `pod install`
+
+#### What did you expected to happen?
+
+> ℹ Please replace this with what you expected to happen.  
+> e.g. Install all pod dependencies correctly.
+
+#### What happened instead?
+
+> ℹ Please replace this with of what happened instead.  
+> e.g. Pod A is missing the subspec B for target C.
+
+### Podfile
+
+> ℹ Please paste the contents of your Podfile below.    
+>  (e.g. via `cat Podfile | pbcopy`)  
+> You can remove this hint then.
+
+```ruby
+# Paste your Podfile contents HERE.
+```