zulip-botserver: Document use of environment var in README.md
```
zulip-botserver --config-file <path to botserverrc> --hostname <address> --port <port>
-
```
Example: `zulip-botserver --config-file ~/botserverrc`
This program loads the bot configurations from the
-config file (botserverrc here) and loads the bot modules.
+config file (`botserverrc`, here) and loads the bot modules.
It then starts the server and fetches the requests to the
above loaded modules and returns the success/failure result.
-Please make sure you have a current botserverrc file with the
-configurations of the required bots.
-Hostname and Port are optional arguments. Default hostname is
-127.0.0.1 and default port is 5002.
+The `--hostname` and `--port` arguments are optional, and default to
+127.0.0.1 and 5002 respectively.
+
+The format for a configuration file is:
+
+ [helloworld]
+ key=value
+ email=helloworld-bot@zulip.com
+ site=http://localhost
+ token=abcd1234
+
+Is passed `--use-env-vars` instead of `--config-file`, the
+configuration can instead be provided via the `ZULIP_BOTSERVER_CONFIG`
+environment variable. This should be a JSON-formatted dictionary of
+bot names to dictionary of their configuration; for example:
+
+ ZULIP_BOTSERVER_CONFIG='{"helloworld":{"email":"helloworld-bot@zulip.com","key":"value","site":"http://localhost","token":"abcd1234"}}' \
+ zulip-botserver --use-env-vars
