update PULL_REQUEST_TEMPLATE and CONTRIBUTING (#3655)

* update PULL_REQUEST_TEMPLATE and CONTRIBUTING

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-Authored-By: Thane Thomson <connect@thanethomson.com>

* note ADRs

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,12 @@
-<!-- Thanks for filing a PR! Before hitting the button, please check the following items.-->
+<!--
 
+Thanks for filing a PR! Before hitting the button, please check the following items.
+Please note that every non-trivial PR must reference an issue that explains the
+changes in the PR.
+
+-->
+
+* [ ] Referenced an issue explaining the need for the change
 * [ ] Updated all relevant documentation in docs
 * [ ] Updated all code comments where relevant
 * [ ] Wrote tests

CONTRIBUTING.md

@@ -4,6 +4,14 @@ Thank you for considering making contributions to Tendermint and related reposit
 
 Please follow standard github best practices: fork the repo, branch from the tip of develop, make some commits, and submit a pull request to develop. See the [open issues](https://github.com/tendermint/tendermint/issues) for things we need help with!
 
+Before making a pull request, please open an issue describing the
+change you would like to make. If an issue for your change already exists,
+please comment on it that you will submit a pull request. Be sure to reference the issue in the opening
+comment of your pull request. If your change is substantial, you will be asked
+to write a more detailed design document in the form of an
+Architectural Decision Record (ie. see [here](./docs/architecture/)) before submitting code
+changes.
+
 Please make sure to use `gofmt` before every commit - the easiest way to do this is have your editor run it for you upon saving a file.
 
 ## Forking