Add readme to nuget package and update README for compatibility with nuget.org

[alistairjevans]

I've updated the README for compatibility with nuget.org, and added it to the package spec. This is how it looks in the nuget upload preview:

Annoyingly, making the README work for nuget.org has possibly made our Github README worse, as follows:

• Images cannot be center aligned.
• I have had to add fixed width/height variants of our logos to the autofac.github.com repository because I can't specify the size in MD.
• The myget and "open in vscode" badges do not work, so I have removed.

I'm contemplating just having a separate README file used only for nuget.org, given how infrequently we change it, so we don't have to take that stuff out. If that seems sensible to others, I will update the PR accordingly.

Closes #1295

[codecov]

Codecov Report

Merging #1308 (8426c26) into develop (f57041c) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1308      +/-   ##
===========================================
- Coverage    76.83%   76.79%   -0.04%     
===========================================
  Files          188      188              
  Lines         5180     5180              
  Branches      1061     1061              
===========================================
- Hits          3980     3978       -2     
- Misses         702      703       +1     
- Partials       498      499       +1     

Impacted Files	Coverage Δ
src/Autofac/Util/SequenceGenerator.cs	71.42% <0.00%> (-28.58%)	⬇️

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[tillig]

It's weird that those other badges don't work. Huh. I mean, I'd have been totally OK if that Gitter badge accidentally got lost 🙈, but it seems weird that MyGet doesn't work.

I think if we have two different READMEs we'll forget that one is there. I'm OK with these minor changes.

I'm guessing it's just the images that won't render - what if we made them just... text links instead of "badges?" I mean, there's technically already a link to the MyGet stuff, so that's taken care of. Maybe under the "Contributing / Pull Requests" there could be something about "or [open it right now in VS Code]".

[alistairjevans]

OK, grammar corrected, VS Code reference added to the end.

[tillig]

🌭