Docs: Add a pull request template (#1239)

Author: tordans

.github/PULL_REQUEST_TEMPLATE.md

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+### Description, Motivation & Context
+
+<!-- Help readers to understand why this is relevant -->
+
+### Related issues
+
+<!-- Please link any related issues here. 
+     Use "Closes #123" to reference issues that should be closed automatically when this is merged. -->
+
+### Links and data
+
+**Relevant OSM Wiki links:**
+- …
+
+**Relevant tag usage stats:**
+> …
+<!-- E.g., Numbers from Taginfo https://taginfo.openstreetmap.org/ and maybe local Taginfo https://taginfo.geofabrik.de/ -->
+<!-- E.g., a link to https://taghistory.raifer.tech -->
+
+### Checklist and Test-Documentation Template
+
+<details><summary>Read on to get your PR merged faster…</summary>
+
+Follow these steps to test your PR yourself and make it a lot easier and faster for maintainers to check and approve it.
+
+**This is how it works:**
+1. After you submit your PR, the system will create a preview and comment on your PR:
+   > 🍱 You can preview the tagging presets of this pull request here.
+   If this is your first contribution to this project, the preview will not happen right away but requires a click from one of the project members. We will do this ASAP.
+
+2. Once the preview is ready, use it to test your changes.
+
+3. Now copy the snippet below into a new comment and fill out the blanks.
+
+4. Now your PR is ready to be reviewed.
+
+```
+## Test-Documentation
+
+### Preview links & Sidebar Screenshots
+
+<!-- Use the preview to find examples, select the feature in question and **copy this link here**.
+     Find examples of nodes/areas. Find examples with a lot of tags or very few tags. – Whatever helps to test this thoroughly.
+     Add relevant **screenshots** of the sidebar of those examples. -->
+
+<!-- FYI: What we will check:
+     - Is the [icon](https://github.com/ideditor/schema-builder/blob/main/ICONS.md) well chosen.
+     - Are the fields well-structured and have good labels.
+     - Do the dropdowns (etc.) work well and show helpful data. -->
+
+### Search
+
+<!-- **Test the search** of your preset and share relevant **screenshots** here.
+     - Test the preset name as search terms.
+     - Also test the preset terms and aliases as search terms (if present). -->
+
+### Info-`i`
+
+<!-- **Test the info-i** for your fields and preset and share relevant **screenshots** here.
+     The info needs to help mappers understand the preset and when to use it.
+     [Learn more…](https://github.com/tordans/id-tagging-schema/blob/main/CONTRIBUTING.md#info-i)
+ -->
+
+### Wording
+
+- [ ] American English
+- [ ] `name`, `aliases` (if present) use Title Case
+- [ ] `terms` (if present) use lower case, sorted A-Z
+<!-- Learn more in https://github.com/openstreetmap/id-tagging-schema/blob/main/GUIDELINES.md#2-design-the-preset -->
+```
+
+</details>