update README install section

I found I had to use add "LinearAlgebraicRepresentation" instead of add("LinearAlgebraicRepresentation") which errored out with ERROR: Could not determine command and wanted to update the README to include also this installation line

[waldyrious]

@grace-harper-ibm, AFAIK the add "LinearAlgebraicRepresentation" line only works in the Pkg REPL (triggered with ]). What they use in the README is the direct method call from the normal Julia REPL. That approach works if one uses the line that's currently commented out in the README, i.e.:

using Pkg
Pkg.add("LinearAlgebraicRepresentation")

I just tested this locally, and it worked as expected. (On an earlier version of this comment, I mentioned getting an "unsatisfiable requirements" error during the process, but that's disappeared now that I've upgraded to Julia 1.6.)