add an issue template for bug reports

This should help users report information we're often missing in bug reports. Ideally, this would be included in a backtrace, but this is simpler to implement.

ISSUE_TEMPLATE.md

@@ -0,0 +1,11 @@
+## Environment
+
+<!-- Please fill in the following information in bug reports, removing the comments like this one in brackets. For feature request, just remove this section. -->
+
+ * Operating system: <!-- Debian GNU/Linux 9.1 (stretch), Windows 7, Ubuntu Xenial, etc -->
+ * Git version: <!-- e.g. 2.11.0, output of git \-\-version (without backslashes) -->
+ * Mediawiki remote version: <!-- version downloaded -->
+ * Install method: <!-- distribution package, CPAN, from source tarball, from git, etc -->
+ * Site URL: <!-- e.g. https://example.com/wiki/... -->
+ * Mediawiki site version: <!-- e.g. Mediawiki 1.25.3 - can be found in https://example.com/wiki/index.php?title=Special:Version -->
+ * Git configuration: <!-- output of `git config -l` -->