Add link to official docs for NAMD once released

[ljwoods2]

[orbeckst]

@amruthesht are there official NAMD docs we can link to or is it the case that our imdclient docs are going to be what's known for NAMD-IMDv3 (partially asking because that's what I am remembering and what I based my comment in PR #114 on).

[amruthesht]

The official documentation will only be out once the new version (NAMD 3.1) is released. However, since we expect users to download NAMD from source and build by registering and gaining access to their GitLab repo, the repository contains user guide-like documentation here - ug/ug_userdef.tex which can be linked in my opinion.