Update readme to explain how to run authenticator executable

[calumcraig]

Current behaviour with the safe-authenticator-cli:

1. Clone GH repo
2. Run cargo build
3. This creates /target/debug/safe_auth or \target\debug\safe_auth.exe (Haven't checked MacOS equivalent.)
4. Simply then running this executable file will fail to connect to the SAFE network as it has no safe_auth.crust.config to provide network details.

(This is in contrast to running using cargo run which sees the config file in the resources directory of the repo and will connect succesfully.)

[calumcraig]

Suggestion:

Update the safe-authenticator-cli readme to reference the above and advise user to make a copy of safe_auth,crust.config inside the /target/debug directory.