README.md: improve introduction

The SCIFIO acronym is now defined, and the fact that SCIFIO focuses on
N-dimensional scientific imaging is explicitly mentioned.
README.md: use spelling "plugin", not "plug-in"

Both are valid but for consistency with ImageJ and SciJava Common, we
leave off the dash.
README.md: be consistent with header markup

You can either use --- and === underlines, or #/##/### header
prefixes. Mixing them is just a recipe for disaster, though!!
README.md: make space between sections consistent
README.md: fix underline lengths

It is not strictly necessary for the underline lengths to match, but it
looks nicer in a text editor.
README.md: use shiny new scif.io website link
README.md: update developer info section

It now includes links to other software libraries compatible with
SCIFIO, including ImgLib2, Bio-Formats and ITK.
README.md: update user info section

It no longer mentions Bio-Formats explicitly, because that is mentioned
in the developer section now. (It was also slightly misleading because
stock Bio-Formats does not yet support SCIFIO formats out of the box;
you need SCIFIO's temporary Bio-Formats fork for that.)

The user section also mentions Fiji, because Fiji now ships with ImageJ2
and can "Switch to Modern Mode" to utilize SCIFIO. Further, we plan for
Fiji's HandleExtraFileTypes to lean on SCIFIO much more heavily soon,
even when in ImageJ 1.x "legacy" mode.
README.md: update mailing list links

We now have a dedicated SCIFIO mailing list!
README.md: tweak "Contributing to SCIFIO" language

Personally I try not to refer to OSS software as "products" because they
are continually improving and actively developed. Nor are they for sale,
but rather intended for the scientific community to freely use and
improve. SCIFIO is a community *project* and as such I prefer that term.
README.md: show console command in fixed width

Using four spaces, GitHub flavored Markdown will show the text in fixed
width font with a cute little box around it. This seems appropriate for
the mvn command.
README.md: make the TestNG template into a link