Merge pull request #3759 from ReactiveX/doc-issue-template

docs(issue): update issue template

Author: kwonoj

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: If rxjs does not behave as expected
+
+---
+
+## Bug Report
+
+**Current Behavior**
+A clear and concise description of the behavior.
+
+**Reproduction**
+- REPL or Repo link:
+(you can use https://stackblitz.com/ to create one to attach here)
+
+```js
+var your => (code) => here;
+```
+
+**Expected behavior**
+A clear and concise description of what you expected to happen (or code).
+
+**Environment**
+- Runtime: [e.g. Node v${x}, Chrome v${x}]
+- RxJS version:
+- (If bug is related) Loader, build configuration: [e.g webpack, angular-cli version, config]
+
+**Possible Solution**
+<!--- Only if you have suggestions on a fix for the bug -->
+
+**Additional context/Screenshots**
+Add any other context about the problem here. If applicable, add screenshots to help explain.

.github/ISSUE_TEMPLATE/documentation.md

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+---
+name: Documentation
+about: Issues around documentation
+
+---
+
+Please check out our documentation repo https://github.com/ReactiveX/rxjs-docs and file issues there.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest new operator or features
+
+---
+
+## Feature Request
+
+With pipable operator, RxJS strongly recommends to create new features via pipable function and publish it as user-land module. If this feature request is
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**(If this is new operator request) describe reason it should be core operator**
+A clear and concise description of reason it can't be achieved via user-land module.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/support-question.md

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+---
+name: Support Question
+about: 'If you have a question, please check out our StackOverflow! '
+
+---
+
+We primarily use GitHub as an issue tracker; for usage and support questions, please check out these resources below. Thanks! 
+
+---
+* StackOverflow: https://stackoverflow.com/questions/tagged/rxjs using the tag `rxjs`
+* Also have a look at the readme for more information, possibly can have some answers already:
+- https://github.com/ReactiveX/rxjs/blob/master/README.md
+- https://github.com/ReactiveX/rxjs/blob/master/MIGRATION.md
+- https://github.com/ReactiveX/rxjs/blob/master/doc/pipeable-operators.md