Markdown is marvellous for early stages of writing just about anything (though esp. handy for technical docs and web content) - you can just ignore formatting and focus on the content & structure. A bit like LaTeX in that regard, dunno if you've used that in the past.
Marvellous for authoring web content since there's lots of support for in the form of HTML converters, plus you can inline any extra HTML if you need. Plus Github is great support for it (
http://github.github.com/github-flavored-markdown/) and it's in fact the most common README syntax in use on Github by now, from what I've seen anyway.
I have a recommendation: try Markdown syntax in combination with the Scrivener writing app (
http://www.literatureandlatte.com). This is the setup I used for my big book thesis. Scrivener can pump out TeX, via a Markdown -> LaTeX converter, and this I used to create a super-pretty ready for print PDF.
Scrivener not that useful for collaborative editing, but great for early solo drafting of especially bigger pieces. It can also spit out an RTF or Word file (plus several other word processor formats) with some basic formatting, so you can generate one of those and E-mail to people (and end up in Highlight Changes Hell if you have more than a couple of others editing!) . Or upload to Google Docs for co-editing, or whatever
.