docs: add issue template

ISSUE_TEMPLATE.md

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+<!--
+Thank you for reporting an issue.
+
+This issue tracker is for bugs and issues found within the JavaScript implementation of libp2p.
+If you require more general support please file an issue on our discuss forum. https://discuss.ipfs.io/
+
+Please fill in as much of the template below as you're able.
+
+Version: package.json version or the commit you have installed.
+Platform: output of `uname -a` (UNIX), or version and 32 or 64-bit (Windows). If using in a Browser, please share the browser version as well.
+Subsystem: if known, please specify affected core module name (e.g Transports, SECIO, etc).
+
+If possible, please provide code that demonstrates the problem, keeping it as
+simple and free of external dependencies as you are able.
+-->
+
+- **Version**:
+- **Platform**:
+- **Subsystem**:
+
+<!-- Bug, Feature, Question, Enhancement, Etc -->
+#### Type:
+
+<!-- 
+One of following:
+  Critical - System crash, application panic.
+  High - The main functionality of the application does not work, API breakage, repo format breakage, etc.
+  Medium - A non-essential functionality does not work, performance issues, etc.
+  Low - An optional functionality does not work.
+  Very Low - Translation or documentation mistake. Something that won't give anyone a bad day.
+-->
+#### Severity:
+
+#### Description:
+
+#### Steps to reproduce the error:
+
+<!--
+This is for you! Please read, and then delete this text before posting it.
+The js-ipfs issues are only for bug reports and directly actionable features.
+
+Read https://github.com/ipfs/community/blob/master/contributing.md#reporting-issues if your issue doesn't fit either of those categories.
+-->