Add README (#23)

* Add README

* Make exit code terminating more clear

README.md

@@ -0,0 +1,29 @@
+# Exercism Canonical Data Syncer
+
+This small application is used to help Exercism tracks keep their exercise-specific `tests.toml` files in sync with the latest canonical data in the [problem-specifications repo](https://github.com/exercism/problem-specifications).
+
+## Usage
+
+The application is a single binary and can be used as follows:
+
+```
+Usage: canonicaldatasyncer [options]
+
+Options:
+  -e, --exercise <slug>        Only sync this exercise
+  -c, --check                  Check if there missing tests. Doesn't update the tests. Terminates with a non-zero exit code if any one or more test cases are missing.
+  -d, --default <mode>         What to do with missing test cases. Allowed values: c[hoose], i[nclude], e[xclude]
+  -o, --verbosity <verbosity>  The verbosity of output. Allowed values: q[uiet], n[ormal], d[etailed]
+  -h, --help                   Show CLI usage
+  -v, --version                Display version information
+```
+
+Running the application will prompt the user to choose whether to include or exclude missing tests. It will update the `tests.toml` file accordingly. If you only want a quick check, you can use the `--check` option.
+
+## Use in your track
+
+To use the application in your track, you can copy the [`scripts/fetch-canonicaldatasyncer`](./scripts/fetch-canonicaldatasyncer) and/or [`scripts/fetch-canonicaldatasyncer.ps1`](./scripts/fetch-canonicaldatasyncer.ps1) files to your track's repository. Running either of these scripts will download the latest version of the `canonicaldatasyncer` tool to your track's `bin` directory.
+
+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/exercism/canonical-data-syncer.

src/arguments.nim

@@ -34,7 +34,7 @@ proc showHelp* =
 
 Options:
   -{ExerciseArgument.short}, --{ExerciseArgument.long} <slug>        Only sync this exercise
-  -{CheckArgument.short}, --{CheckArgument.long}                  Check if there missing tests. Doesn't update the tests
+  -{CheckArgument.short}, --{CheckArgument.long}                  Check if there missing tests. Doesn't update the tests. Terminates with a non-zero exit code if any one or more tests are missing.
   -{DefaultArgument.short}, --{DefaultArgument.long} <mode>         What to do with missing test cases. Allowed values: c[hoose], i[nclude], e[xclude]
   -{VerbosityArgument.short}, --{VerbosityArgument.long} <verbosity>  The verbosity of output. Allowed values: q[uiet], n[ormal], d[etailed]
   -{HelpArgument.short}, --{HelpArgument.long}                   Show CLI usage