create example for field level encryption

In support of users of the devguide-examples and possible high-level conceptual documentation and other samples being created, add on master branch of devguide-examples two examples. They may be in the same code file.

First, an example that shows how to encrypt a field in a document, store it and read it back.

Second, given a specific document and the necessary keys and other information, show how one would read encrypted fields. It's expected that for this one, the keys would be in files in standard formats in the repository. The same set of files should work for all SDK implementations.

Use the .NET example as a pattern.

If this does not make sense for lcb, then at least add a README that describes why it is not present. e.g.: field_encryption-README.