Make each_record take a description

Change the `each_record` syntax to take a description (parallel to the field name in `to_field`) for self-documenting indexing code and to make the description available for logging/debugging in the `context.field_name` within the applied lambda.

```ruby

to_field('id) ->(a,b,c) { #do whatever }
each_record("Prep titles for title work") do ...end
to_field('title_ab') ->(a,b,c) { ... }

```


Changed syntax, tests, and the README.md file.