Convert readme.txt into Markdown

As an active user of ANSICON, I remember that it took me some time to get it up and running.  Just today, my anti-virus software complained about the (now obsolete) `ANSI-LLW.exe`.  While updating to the latest version, I noticed that the readme is still "suboptimal", forcing me to remember + revisit how it works.

I'd like to improve the documentation, modernize it, and make it user-friendly.

This PR is not complete yet. — I did not want to spend too much time on it, prior to getting an initial confirmation that moving from `readme.txt` to `readme.md` will be accepted.  In case it is, I will add further commits to this PR based on your feedback, and squash/rebase the branch before it is merged into the mainline.

A `readme.md` file is automatically discovered by GitHub and displayed (formatted) when visiting the main page of the repository.  That is the very first page a user looks at, so it is the most important place to "sell" and document a product/application.

1. The vast majority of the file stays the same; mainly some indentation + quote-style changes.
1. The 3 chapters about ANSI sequences are wrapped into a new main _Sequences_ chapter.
1. The _Limitations_ chapter is moved before the _Sequences_ chapter, because it is more important for users and first-time evaluators.
1. The initial copyright/license information is moved to the bottom (_Distribution_).  The copyright year is updated for 2014.

Additional notes:

1. I noticed that ANSICON leverages binary attachments on GitHub releases since 1.66. :+1:, excellent.
1. That aspect enables us to modernize further, leveraging http://shields.io.  The badge not only looks fancy, it also avoids having to update the version number in the readme.

Possible improvements (not included yet):

1. The short description at the top should be immediately followed by a long description.  The long description should be user-oriented; i.e., mainly explaining _why_ anyone would want to install and use this software and what the benefit is of doing so.
1. The primary _Installation_ chapter needs more structure/formatting and should include quick-start instructions to set up ANSICON as quickly as possible for the most common use-case.
1. The _Usage_ chapter could use a clarification upfront that _installing_ will globally enhance ComSpec, so "usage" becomes implicit by starting a cmd shell.
1. Available command line options could be reformatted as a table, and their descriptions could use some grammar love + more elaborate explanations.  Alternatively, the section may be replaced entirely by a short pointer to `ansicon --help`.
1. The version changelog should be moved into a separate file, or removed altogether (in favor of GitHub release notes).
1. The _Sequences_ chapter(s) _could_ be moved into the existing `sequences.[txt|md]` file, so as to stop cluttering the main readme with that deeply internal/technical information.

Thoughts?