Expand details on how to keep the `configuration.js` file secure

[manv3lez]

The configuration.js file is currently created at the root of the directory of new instances of the dctrl-admin app, as described in the README.md.

We are aware that the information in this file is sensitive. As we encourage other communities to create new instances of the app, we should show them how to best handle this file, and the information in it.

[manv3lez]

The following details may be relevant to a new instance:

• How to properly use the local bitcoind wallet to generate the secrets
• How to properly use a physical hardware wallet (Trezor, others) to properly secure them among the community members