GitHub pull request template

This commit makes use of GitHub's feature to define multiple pull
request templates (1). See the GitHub Help (2) for more details about
issue and pull request templates.

References:
  (1) blog.github.com/2016-02-17-issue-and-pull-request-templates
  (2) help.github.com/articles/about-issue-and-pull-request-templates

Epic: GH-33
Depends on GH-49
Resolves GH-42

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Click on the "Preview" tab to render the instructions in a more readable format -->
+
+> **Please read the [contribution guidelines](https://github.com/arcticicestudio/snowsaw/blob/develop/CONTRIBUTING.md) before filling out this issue template**.
+
+## Prerequisites
+
+This section and the instructions in the sections below are only part of this pull request template. Please ensure to **delete this whole section, all pre-filled instructions of the sections below and sections you have not filled out before submitting** to ensure a clear structure and overview.
+
+Please do your best to provide as much information as possible and use a clear and descriptive title for your enhancement suggestion or bug fix to help maintainers and the community understand and reproduce the behavior, find related pull requests and to merge it faster.
+
+- **Ensure the pull request has not already been submitted by using the [GitHub Issue search](https://github.com/arcticicestudio/snowsaw/issues)** — check if this enhancement or bug fix has already been suggested. If it has **and the pull request is still open**, add your additions as comment or [suggest a change](https://help.github.com/articles/incorporating-feedback-in-your-pull-request/#applying-a-suggested-change) to the existing pull request instead of opening a new one. If you find a closed pull request that seems to be similar to this one, include a link to the original pull request in the [metadata head](#metadata-head) section of this new pull request.
+- **Ensure to adhere to the [pull request contribution guidelines](https://github.com/arcticicestudio/snowsaw/blob/develop/CONTRIBUTING.md#pull-requests)**, especially the one for tests and documentations.
+- **Check if the enhancement has already been implemented or bug already fixed** — use the [latest version](https://github.com/arcticicestudio/snowsaw/releases/latest) and [`develop`](https://github.com/arcticicestudio/snowsaw/tree/develop) branch to ensure that the enhancement or bug fix has not already been added.
+
+## Metadata Head
+
+The metadata head should be added to the top of the pull request as [Markdown text quote](https://help.github.com/articles/basic-writing-and-formatting-syntax) containing the [GitHub issue keyword(s)](https://help.github.com/articles/closing-issues-using-keywords) to link to the related enhancements suggestions (`Closes`) or bug reports (`Fixes`). You can add additional details like dependencies to other pull requests and the order it needs to be merged.
+
+> Closes ISSUE_ID  
+> Must be merged **after**/**before** ISSUE_ID
+
+## Description
+
+Describe the changes as in many relevant details as possible. If this is a enhancement suggestion add specific use-cases and explain why this feature or improvement would be useful. If this is a bug fix ensure to provide a _before/after_ comparison by describing the current behavior and the new behavior.
+
+## References
+
+Add any other references and links which are relevant for this pull request.