Add default pull request template (braintreeps#89)

PULL_REQUEST_TEMPLATE.md

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+Please use the following structure when proposing changes to our shared Vim configuration and make sure to complete the checklist at the end.
+
+# What
+
+Summarize your change.
+
+_Example: Specify full path when running focused specs._
+
+# Why
+
+Summarize the motivation for the change (bug, feature, etc.)
+
+_Example: When only a relative path is specified, focused specs don't run if the shell's current working directory differs from the project root._
+
+# Checklist
+
+- [ ] Send RFC email to Braintree developers _if change may be impactful_. Please include a link to this PR so that disucssion about the pros and cons of the change remains linked to the proposed changes. **Recent examples include**: addition of linter and completer, no longer removing end-of-line whitespace on save, changing to fzf for file lookup.