string-solver-integration doc update

[natsteven]

The current string-solver-integration.md in docs for string-support-dev branch does not explain the need for including the solver jar as part of the jpf-symbc.native_classpath in jpf.properties. I would have found this helpful when integrating a solver.

[sohah]

Hi @natsteven , thank you for opening this issue. Can you make the necessary changes in a pull request?