GitHub supports several lightweight markup languages for documentation; the most popular ones (generally, not just at GitHub) are Markdown and reStructuredText. Markdown is sometimes considered easier to use, and is often preferred when the purpose is simply to generate HTML. On the other hand, reStructuredText is more extensible and powerful, with native support (not just embedded HTML) for tables, as well as things like automatic generation of tables of contents.
Many people naively assume an ordinary .NET HashSet preserves insertion order. Indeed HashSet accidentally preserves insertion order until you remove and re-add some elements. There is such a data structure in Java - LinkedHashSet which respects order and has O(1) RW times.
No, I did not find a (working) corresponding implementation in .NET. That's I wrote this one.
The implementation uses linked list in combination with dictionary to define the iteration, ordering and at the same time allow O(1) removal.
The order is not affected if an element is re-inserted into the set it retains it's old position.