Add a template for Pulp-Smash issues.

In order to create a standard, and make easy to a new contributor to
file an issue it was included a template with the the basics steps in
how to file a pulp-smash issue.

It was added a dir `.github` to the root system of the project, thus
avoiding to add extra files to the root system of smash.

Futures templates can be added to the same dir as well.

.github/ISSUE_TEMPLATE.md

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+
+1. Describe what you expected to happen.
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+2. Describe what actually happened.
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+3. Describe what version of Pulp and what OS is affected by the issue.
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+4. State steps to re-create the issue.
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+5. If there is a related pulp.plan.io issue, link to it, and set the "smash test" field.
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+6. Choose proper labels. See: [Smash Labels](https://pulp-smash.readthedocs.io/en/latest/about.html#labels)