Add basic "Limitations" section information

[Sypherd]

Per the ACL Rolling Review page, a "Limitations" section is now required. As noted in #39, the exclusion of a required section in the example LaTeX files can be a bit confusing. I do recognize that this has been discussed before (see #18 (comment)) and has been excluded to distinguish between style and content guidelines. Since the "Limitations" section is now mandatory and has specific requirements (e.g., placement: "at the end of the paper, before the references"; verbiage: 'titled “Limitations”'; length: "up to one page"), I would argue that (at the very least) placement information and section title specification could be considered style guidelines. I've included a small amount of additional information which could certainly be trimmed down the barebones stylistic requirements. Just wanted to bring this back up for discussion.