Add a git submodule for the marc2bibframe converter

[dazza-codes]

The conversion script in this project is using an LC converter, i.e.

• https://github.com/lcnetdev/marc2bibframe

This might be included in this repository as a git submodule. The advantages to this would be:

• [ ] using the git submodule features to track a specific version of the LOC converter
• [ ] create an install.sh script to install scripts in this package and the LOC converter

A similar git submodule approach could be adopted for other converters available in public git repositories, e.g. the LD4* converter is on github.

[dazza-codes]

See notes about how to use the git submodule on deployment at

• https://github.com/dpn-admin/dpn-server/wiki/Deployment-for-dpn-server

[dazza-codes]

Update the README for this project with notes about how to use the git submodules.

Include a note about updating the submodule to the latest master for the remote, e.g.

$ cd ld4p-tracer-bullet-scripts # a clone of this project
$ git submodule foreach git pull origin master

[dazza-codes]

This is completed by #12 and #16