Add pull request template (#110)

.github/pull_request_template.md

@@ -0,0 +1,11 @@
+<!--
+ Thank you for opening a pull request to Tapir.jl!
+ Please note that this project operates the following policy: any time a PR is merged
+ which modifies code in the src directory, a release must be made.
+
+ Consequently, if your PR modifies something in src, please follow semver to figure out
+ how you should modify the version number in Project.toml. If it's unclear to you how the
+ version number should be modified, please open your PR without modifying it, and ask for
+ assistance -- one of the maintainers will be happy to help figure out what the new
+ version number ought to be.
+-->