Add a git submodule for the DLSS tracer bullets converter(s)

[dazza-codes]

There are conversion scripts in this project that depend on another project, i.e.

• https://github.com/sul-dlss/ld4p-tracer-bullets

This can be included in this repository as a git submodule. The advantages to this are:

• [ ] using the git submodule features to track a specific version of the other project
• [ ] create an install.sh script to install the dependency for this project

[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