Extract `PEER_OFFLINE_LIMIT` into `config.json`

[`PEER_OFFLINE_LIMIT`](https://github.com/Automattic/jetmon/blob/master/lib/jetmon.js#L26) in `jetmon.js` determines how many peers have to confirm that the site is down before a notification email is sent.
By default it's 3, however on our local envs we only run a single veriflier service, therefore we need to set this to `1` to actually send the monitoring data to WPCOM.

We need to:
- extract this setting into `config.json`
- add this information into `README.md`

Start at lib/jetmon.js line 26 to trace how PEER_OFFLINE_LIMIT is read and used, then inspect the existing config.json structure. Move the setting into config.json with the required default and document it in README.md; done means the configured value controls notifications and the README explains the option.

Written by the indexing model from the issue text.