added usage

src/i18n/compare.js

@@ -1,5 +1,27 @@
 #!/usr/bin/env node
 const arg = process.argv[2]
+
+if (typeof arg === 'undefined') {
+  console.log('This is a very simple and tiny tool that checks en.json with any other language and')
+  console.log('outputs all the things present in english but missing in foreign language.')
+  console.log('')
+  console.log('Usage: ./compare.js <lang> ')
+  console.log('       or')
+  console.log('       node ./compare.js <lang>')
+  console.log('')
+  console.log('Where <lang> is name of .json file containing language. For ./fi.json it should be:')
+  console.log('      ./compare.js fi ')
+  console.log('')
+  console.log('Limitations: ')
+  console.log('* This program does not work with languages left over in messages.js')
+  console.log('* This program does not check for extra strings present in foreign language but missing')
+  console.log('  in english.js (for now)')
+  console.log('')
+  console.log('There are no other arguments or options. Make an issue if you encounter a bug or want')
+  console.log('some feature to be implemented. Merge requests are welcome as well.')
+  return
+}
+
 const english = require('./en.json')
 const foreign = require(`./${arg}.json`)