Document ag dependency

[foolip]

Trying to run node ./main/selenium_og.es6.js with prerequisite steps done, I get:

throttle()ing 1 promisers, 5 at a time
Starting throttle() promiser 0 as active 0
{"browserName":"Edge","browser_version":"18.0","os":"Windows","os_version":"10","resolution":"1024x768"}
LOGG Creating driver
Started throttle() promiser 0 as active 0
throttle() wait
Build with { name: 'JS Object Graph Data Collection (window...)',
  browserName: 'Edge',
  browser_version: '18.0',
  os: 'Windows',
  os_version: '10',
  resolution: '1024x768',
  'browserstack.local': true,
  'browserstack.user': 'philipjgenstedt2',
  'browserstack.key': 'SECRET',
  'browserstack.video': true,
  project: 'web-apis-investigator',
  build: '0.0.1:master@589e7758bf0c566aaf846b1773f9a8aa53ebb1e3' } https://philipjgenstedt2:SECRET@hub.browserstack.com/wd/hub
throttle() promiser 0 complete
All throttle() promisers complete. Resolving...
EXIT: g-zipping logs
EXIT: g-zipping done
EXIT: moving logs to /Users/foolip/src/web-apis/.log/latest
/bin/bash: ag: command not found
usage: mv [-f | -i | -n] [-v] source target
       mv [-f | -i | -n] [-v] source ... directory
child_process.js:651
    throw err;
    ^

Error: Command failed: mv $(ls "/Users/foolip/src/web-apis/.log" | ag -v '(latest|.tar.gz)') "/Users/foolip/src/web-apis/.log/latest/"
/bin/bash: ag: command not found
usage: mv [-f | -i | -n] [-v] source target
       mv [-f | -i | -n] [-v] source ... directory

    at checkExecSyncError (child_process.js:611:11)
    at execSync (child_process.js:648:13)
    at process.once.status (/Users/foolip/src/web-apis/lib/logger.es6.js:63:3)
    at Object.onceWrapper (events.js:273:13)
    at process.emit (events.js:182:13)

The first sign of trouble is "/bin/bash: ag: command not found".