docs: add an issue template

ISSUE_TEMPLATE.md

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+#### What version of ripgrep are you using?
+
+Replace this text with the output of `rg --version`.
+
+#### What operating system are you using ripgrep on?
+
+Replace this text with your operating system and version.
+
+#### Describe your question, feature request, or bug.
+
+If a question, please describe the problem you're trying to solve and give
+as much context as possible.
+
+If a feature request, please describe the behavior you want and the motivation.
+Please also provide an example of how ripgrep would be used if your feature
+request were added.
+
+If a bug, please see below.
+
+#### If this is a bug, what are the steps to reproduce the behavior?
+
+If possible, please include both your search patterns and the corpus on which
+you are searching. Unless the bug is very obvious, then it is unlikely that it
+will be fixed if the ripgrep maintainers cannot reproduce it.
+
+#### If this is a bug, what is the actual behavior?
+
+Show the command you ran and the actual output. Include the `--debug` in your
+invocation of ripgrep if possible.
+
+#### If this is a bug, what is the expected behavior?
+
+What do you think ripgrep should have done?