Update README with usage strategy

[ronaldtse]

From @dadooda

Explain strategy first, add concrete details as tool functionality gets implemented.

The “strategy” doesn’t change over time and is roughly like this:

1. In the project, create fields for new encrypted values using regular Rails migration(s).
2. Run stand-alone Transcryptor against the prepared database(s). This step should fail safe.
3. Once everything settles, create migration(s) to remove old encrypted fields/data.

[ronaldtse]

The point is there are two use cases of transcryptor and we should separate them.

1. Infrequent batch use: migrating from one encryption scheme to another one, maybe only once. There's no need to include transcryptor in the bundle.
2. Automated re-encryption: Clean and understandable model for recurring re-encryptions #12