Add issue template

.github/issue_template.md

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+<!--
+
+Thanks for opening an issue!
+
+If you're reporting an error thrown by SwiftyRSA, please include the following details:
+
+ - Version of SwiftyRSA in Podfile/Cartfile
+ - Exact code that you're using to create the key / message / signature
+ - The full description of the error (`error.localizedDescription`)
+ - Contents of the key / message / signature:
+   - If content is already a base 64 string, share this
+   - If content is a PEM file, share the file content
+   - If content is a DER file, run `base64 -i PATH_TO_FILE` and share the output
+ 
+*Important:* Do not share the contents of your private keys, unless they're non-sensitive.
+
+Note: This is an HTML comment, you can remove it altogether and/or write outside of it.
+-->