PR template: ask for fn docs and tests

.github/PULL_REQUEST_TEMPLATE

@@ -3,6 +3,19 @@ Don't use past tense (e.g. "fixed foo bar").
 
 Explain what your PR does and why.
 
+If you are adding a new function, please document it and add tests:
+
+```
+// foo does foo and bar
+fn foo() {
+
+// file_test.v
+fn test_foo() {
+    assert foo() == ...
+    ...
+}
+```
+
 Before submitting a PR, please run the tests with `make test`, and make sure V can still compile itself. Run this twice:
 
 ./v -o v