#269: Update CONTRIBUTING guidelines, add PR and issue templates

- Add issue templates for bugs, translations and new snippets

- Add PR templates for general purpose, new snippets and translations

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: Bug report
+about: Bug template
+title: 'Fix ...'
+labels: 'bug'
+
+---
+
+<!--
+Hi, thanks for submitting a bug. We appreciate that.
+
+But, we will need some information about what's wrong to help you.
+-->
+## What's wrong
+
+<!-- Describe what is not working. -->
+
+## How it should work?
+
+<!-- Describe how it should work. -->
+
+## Checklist before calling for maintainers
+
+* [ ] Have you checked to ensure there aren't other open [Issues](../../../issues) for the same problem?
+

.github/ISSUE_TEMPLATE/new_snippet.md

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+---
+name: New snippet
+about: New snippet template
+title: 'New snippet: ...'
+labels: 'new snippets'
+---
+
+<!--
+Hi, thanks for submitting a new snippet. We appreciate that.
+
+But, we will need some information and (optionally) explanation to accept it.
+-->
+## Description
+
+## Snippet preview
+
+## Checklist before calling for maintainers
+
+* [ ] Have you checked to ensure there aren't other open [Issues](../../../issues) for the same update/change?
+* [ ] Have you checked that this snippet is not similar to any of the existing snippets?
+<!--Explanation is optional. You may suggest a snippet without deepp understanding of its behavior.-->
+* [ ] Have you added an `Explanation` section? It shall include the reasons of changes and why you'd like us to include them
+

.github/ISSUE_TEMPLATE/translation.md

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+---
+name: Translation
+about: New translation template
+title: 'Tranlate to ...'
+labels: 'translations'
+
+---
+
+<!--Hi, thanks for suggesting a new translation. We appreciate that.-->
+## Checklist before calling for maintainers
+
+* [ ] Have you checked to ensure there aren't other open [Issues](../../../issues) for the same tranlation?
+* [ ] Do you wish to make a translation by yourself?

.github/PULL_REQUEST_TEMPLATE/new_snippet.md

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+## #(issue number): Summarize your changes
+
+<!--- This project only accepts pull requests related to open issuesIf
+Please discuss all questions in an issue first -->
+<!--- Special phrase to auto-close the issue that your PR fixes -->
+Closes # (issue number)
+
+## Checklist before requesting a review
+
+- [ ] Have you written simple and understandable explanation?
+- [ ] Have you added new snippet into `snippets/` with suitable name and number?
+- [ ] Have you updated Table of content? (later will be done by pre-commit)
+- [ ] Have you followed the guidelines in [CONTRIBUTING.md](../../CONTRIBUTING.md)?
+- [ ] Have you performed a self-review?
+- [ ] Have you added yourself into [CONTRIBUTORS.md](../../CONTRIBUTORS.md)?

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+## #(issue number): Summarize your changes
+
+<!--Please include the reasons behind these changes and any relevant context.
+This project only accepts pull requests related to open issues -->
+<!--- Special phrase to auto-close the issue that your PR fixes -->
+Closes # (issue number)
+
+## Checklist before requesting a review
+
+- [ ] Have you followed the guidelines in [CONTRIBUTING.md](../../CONTRIBUTING.md)?
+- [ ] Have you performed a self-review?
+- [ ] Have you added yourself into [CONTRIBUTORS.md](../../CONTRIBUTORS.md)?
+

.github/PULL_REQUEST_TEMPLATE/translation.md

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+## #(issue number): Translate to ...
+
+<!--- This project only accepts pull requests related to open issues -->
+<!--- Special phrase to auto-close the issue that your PR fixes -->
+Closes # (issue number)
+
+## Checklist before requesting a review
+
+- [ ] Have you fetched the latest `master` branch?
+- [ ] Have you translated all snippets?
+- [ ] Have you followed the guidelines in [CONTRIBUTING.md](../../CONTRIBUTING.md)?
+- [ ] Have you performed a self-review?
+- [ ] Have you added yourself into [CONTRIBUTORS.md](../../CONTRIBUTORS.md)?