Cleanup README (add pact-lsp, remove stack instructions)

[rsoeldner]

This PR adds a note on the Pact Language Server.
Additionally it removes the stack building instructions (the corresponding stack.yaml file does not exists anymore).

PR checklist:

• Test coverage for the proposed changes
• PR description contains example output from repl interaction or a snippet from unit test output
• Documentation has been updated if new natives or FV properties have been added. To generate new documentation, issue cabal run tests. If they pass locally, docs are generated.
• Any changes that could be relevant to users have been recorded in the changelog
• In case of changes to the Pact trace output (pact -t), make sure pact-lsp is in sync.

Additionally, please justify why you should or should not do the following:

• Confirm replay/back compat
• Benchmark regressions
• (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact

[emilypi]

LGTM

[imalsogreg]

LGTM!