Add a PR checklist to melange.

This introduces a PR checklist to `melange` to remind folks that being able to rebuild Wolfi is the primary feature of `melange`, and that changes shouldn't be make that risk our ability to keep building Wolfi successfully.

Signed-off-by: Matt Moore <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

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+## Melange Pull Request Template
+
+<!--
+*** PULL REQUEST CHECKLIST: PLEASE START HERE ***
+
+The single most important feature of melange is that we can build Wolfi.
+
+Many changes to melange introduce a risk of breaking the build, and sometimes
+these are not flushed out until a package is changed (much) later.  This
+pertains to basic execution, SCA changes, linter changes, and more.
+-->
+
+### Functional Changes
+
+- [ ] This change can build all of Wolfi without errors (describe results in notes)
+
+Notes:
+
+### SCA Changes
+
+- [ ] Examining several representative APKs show no regression / the desired effect (details in notes)
+
+Notes:
+
+### Linter
+
+- [ ] The new check is clean across Wolfi
+- [ ] The new check is opt-in or a warning
+
+Notes: