Add a template describing issue requirements

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,55 @@
+This template is mostly a guideline, not a strict requirement, except
+the sections marked `(required)`.
+
+Please make sure that the issue is specific to the plugin for sbt that is
+observable only using the latest version of `sbt-scalafmt` and does not
+appear if you use the `scalafmt` **CLI** using the same `.scalafmt.conf`:
+`https://scalameta.org/scalafmt/docs/installation.html#cli`
+
+Prior check via the CLI is _mandatory_.
+
+## Configuration (required) ##
+
+Please paste the values of all `sbt-scalafmt`
+[settings](https://scalameta.org/scalafmt/docs/installation.html#settings)
+found in your `build.sbt` file here:
+```
+scalafmtConfig = <please enter the file location here>
+...
+```
+
+Please paste the contents of your `.scalafmt.conf` file (mentioned above
+under `scalafmtConfig` sbt setting) here:
+```
+version = <please enter the version here and make sure it's the latest>
+...
+```
+
+NB: before submitting, please confirm that the problem is observed in
+the *latest published* version of the sbt plugin! We don't publish
+hotfixes for older versions, and the problem you have observed in an
+older version may have already been fixed.
+
+## Command-line parameters (required) ##
+
+When I run sbt-scalafmt via `sbt` like this: `<sbt command-line parameter>`
+
+## Problem (required)
+
+`sbt-scalafmt` fails to format files mentioned below, or issues an
+error as follows:
+```scala
+OUTPUT FROM SBT-SCALAFMT
+```
+
+## Expectation
+
+...
+
+## Workaround
+
+I've found that by...
+
+## Notes
+
+See also...