Add version-string (descriptive version strings for Common Lisp)

[atgreen]

version-string generates descriptive, git-describe-style version strings for
Common Lisp projects, so applications can report a meaningful version
(including tag, commit count, and short hash) at runtime. Single-file library
with no dependencies.

Sources: https://github.com/atgreen/cl-version-string
Docs/README: https://github.com/atgreen/cl-version-string#readme
License: MIT
Primary system: version-string

Suggested source.txt (proposed project name: version-string):
latest-github-release https://github.com/atgreen/cl-version-string.git