Add shared help and tests doc (#1557)

Author: ErikSchierboom

config/exercise_readme.go.tmpl

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+# Help
+
+To get help if you're having trouble, you can use one of the following resources:
+
+- [Gitter](https://gitter.im/exercism/xcsharp) is Exercism C# track's Gitter room; go here to get support and ask questions related to the C# track.
+- [/r/csharp](https://www.reddit.com/r/csharp) is the C# subreddit.
+- [StackOverflow](http://stackoverflow.com/questions/tagged/c%23) can be used to search for your problem and see if it has been answered already. You can also ask and answer questions.

exercises/shared/.docs/cli.md

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+# Tests
+
+You can run the tests by opening a command prompt in the exercise's directory, and then running the [`dotnet test` command](https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-test)
+Alternatively, most IDE's have built-in support for running tests, including [Visual Studio](https://docs.microsoft.com/en-us/visualstudio/test/run-unit-tests-with-test-explorer), [Rider](https://www.jetbrains.com/help/rider/Unit_Testing_in_Solution.html) and [Visual Studio code](https://github.com/OmniSharp/omnisharp-vscode/wiki/How-to-run-and-debug-unit-tests).
+See the [tests page](https://exercism.io/tracks/csharp/tests) for more information.
+
+## Skipped tests
+
+Initially, only the first test will be enabled.
+This is to encourage you to solve the exercise one step at a time.
+Once you get the first test passing, remove the `Skip` property from the next test and work on getting that test passing.