GITHUB: Add Pull Request template

.github/pull_request_template.md

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+
+Thank you for contributing to ScummVM. Please read the following carefully before submitting your Pull Request.
+
+Make sure your individual commits follow the guidelines found in the ScummVM Wiki: https://wiki.scummvm.org/index.php?title=Commit_Guidelines. If they're not please edit them before submitting the Pull Request.
+
+Commits and Pull Requests should use the following template:
+
+```
+SUBSYSTEM: Short (50 chars or less) summary of changes
+
+More detailed explanatory text, if necessary.  Wrap it to about 72
+characters or so.  In some contexts, the first line is treated as the
+subject of an email and the rest of the text as the body.  The blank
+line separating the summary from the body is critical (unless you omit
+the body entirely); tools like rebase can get confused if you run the
+two together.
+
+Write your commit message in the present tense: "Fix bug" and not "Fixed
+bug."  This convention matches up with commit messages generated by
+commands like git merge and git revert.
+
+Further paragraphs come after blank lines.
+
+- Bullet points are okay, too
+
+- Typically a hyphen or asterisk is used for the bullet, preceded by a
+ single space, with blank lines in between, but conventions vary here
+
+- Use a hanging indent
+```