Refine the install section in README

[augustobmoura]

After npm/cli#2872 I thought it would be more useful to have a more concise install section

What I did:

• Linked asdf-core repository in the install section
• Pushed the OpenPGP issue with older releases to it's own section, after Add check to see if key already exists locally before querying keyservers #209 it's not really important for installation

[augustobmoura]

@Stratus3D I'm merging this PR, because is just some minors adjustments in README, tagging you just for information

[Stratus3D]

Thanks @augustobmoura ! Changes here look good. Sorry I didn't merge it for you. One thing to note that you might consider reverting:

There is some inherit risk for users in installing and running plugins on their system. One of the ways I try to reduce the risk is by making it clear what is going on behind the scenes. We've intentionally kept the long plugin-add syntax in readmes and other docs because it clearly shows where the code is coming from (users sees it's a git repo named asdf-vm/asdf-nodejs):

asdf plugin-add nodejs https://github.com/asdf-vm/asdf-nodejs.git

This syntax also allows the user to easily change the name to something other than nodejs by specifying any other value.

[augustobmoura]

Makes sense to me, I will open a new PR to revert this change