Improve pull request template

- Make it clear that we maintainers are volunteers and have limited time.
- Tell contributors that a dimensioned drawing of the asset would help a lot (with link to tutorial)

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,3 +11,8 @@ Thanks for creating a pull request to contribute to the KiCad libraries! To spee
 - [ ] If there are matching symbol or 3D model pull requests, provide link(s) as appropriate
 - [ ] Check the output of the Travis automated check scripts - fix any errors as required
 - [ ] Give a reason behind any intentional library convention rule violation.
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+---
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+Be patient, we maintainers are volunteers with limited time and need to check your contribution against the datasheet. You can speed up the process by providing all the necessary information (see above). And you can speed up the process even more by providing a dimensioned drawing of your contribution. A tutorial on how to do that is found here: https://forum.kicad.info/t/how-to-check-footprint-correctness/9279 (This is optional!)
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