Update README.md to link to demo repo

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

After a request by @anutosh491 here compiler-research/xeus-cpp-wasm#2 I added a link to the demo repo in xeus-cpps main repo readme . I kept the one linking to the deployment we get from building from source, and emphasis that the demo repo shows more of the capabilities, and is based on the latest release version of xeus-cpp.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (951e670) to head (6f9015a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #355   +/-   ##
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  Coverage   81.78%   81.78%           
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  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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[anutosh491]

I think this is good. Let's see if we get any other reviews here.

[vgvassilev]

lgtm!