Add deprecation note to README

[rubensworks]

As discussed in https://github.com/LinkedDataFragments/jQuery-Widget.js/issues/13, this adds a deprecation note to the README.

Others things we need to do after merging this:

• Add a deprecation warning in the repo description.
• Remove it from the LDF website(?)
• Add a deprecation warning to the npm package.
• Archive the GitHub repo.

[rubensworks]

Add a deprecation warning to the npm package.

@RubenVerborgh Could you do this one ^ ?

Something like the following should work:

$ npm deprecate ldf-client "deprecated in favor of @comunica/actor-init-sparql"