ENH: update and reformat README

[ncullen93]

This proposed PR updates and reformats the README. It is best to view it visually on my fork at https://github.com/ncullen93/ANTs instead of the raw file.

Improvements:

• You don't have to leave the readme to learn how to install ANTs in a basic scenario.
• All learning resources are grouped together so when I’m in “learning mode” I can quickly evaluate which one I need.
• Same with scripts / example code that are now grouped a bit more semantically.
• For references, article titles are displayed so I can quickly evaluate which are relevant rather than having to follow a ton of links that take me away from the readme.
• In general, it's much clearer now when you're nevigating to a script, to a tutorial, or to a journal article. It was a complete lottery what I’m going to get from the links on the current Readme.
• In general, links that take users away from the repo should be minimized… they’ll get distracted and never come back.
• Links to outdated (e.g., source forge) or unfinished resources are all removed

[ntustison]

Hey @ncullen93 , I think that looks very good. I would obviously update the team list to include yourself and probably level the listing so that everybody's in the bulleted list or in a single sentence. I'd probably also add a recent paper but I can do that after your merge.

[stnava]

thanks for this

perhaps link to the wiki some where https://github.com/ANTsX/ANTs/wiki

[ncullen93]

@ntustison I updated the team section and agree it would be nice for you to update or add references eventually.

@stnava Great idea - the ANTs Wiki is now the first link under the "Learning Resources" section.

Feel free to give more feedback or merge and then make any changes you want.

[cookpa]

Merging in anticipation of next release (ITK 5.4.0 is about to be announced)