Link to JSON RPC specification in the README

[Seelengrab]

Clarifies what is supported by this package and links to the specification.

I don't think the VS Code specific docs need to be changed or mentioned in the changelog.

For every PR, please check the following:

• End-user documentation check. If this PR requires end-user documentation in the Julia VS Code extension docs, please add that at https://github.com/julia-vscode/docs.
• Changelog mention. If this PR should be mentioned in the CHANGELOG for the Julia VS Code extension, please open a PR against https://github.com/julia-vscode/julia-vscode/blob/master/CHANGELOG.md with those changes.

[codecov]

Codecov Report

Merging #69 (39229a5) into master (5adc04b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #69   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files           4        4           
  Lines         246      246           
=======================================
  Hits          225      225           
  Misses         21       21           

Flag	Coverage Δ
unittests	91.46% <ø> (ø)

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[Seelengrab]

Coverage decreasing in files I didn't touch? Seems like a fluke :)