Merge pull request apple#681 from thomasvl/template

Add a template for new issues.

Author: thomasvl

.github/issue_template.md

@@ -0,0 +1,19 @@
+Please be sure to include:
+
+* what OS you are developing on (Linux or macOS, including the version)
+
+* for macOS, what version of Xcode you are using (`xcodebuild -version`),
+  for Linux, what version of Swift (`swift --version`)
+
+* what version of Swift is your code set to compile with (i.e. from project
+  settings, etc.)
+
+* what branch/tag of SwiftProtobuf you are using (1.0.0, etc.)
+
+* if you are getting compile errors, please be sure include all errors/warnings,
+  sometimes the error before the one you are stuck on is important.
+
+* lastly, if it all possible, provide a snippet of `.proto` and/or source
+  that shows the problem.
+
+Thank you!