Fix golangci-lint installation

[GusAntoniassi]

Hello, I wasn't able to use this action because the golangci-lint tool apparently wasn't being in the PATH.

I have done some debugging, ran some ls -lah inside the job, and compared with the actions/setup-go installer, and found out the tool was being extracted in a golangci-lint-${version}-linux-amd64 directory.

I have added the following code to add the correct directory to PATH:

  // Tool will be extracted in a directory with the compressed file name
  toolRoot = path.join(toolPath, `golangci-lint-${version}-linux-amd64`)

  // Add golangci-lint dir to bin
  core.addPath(toolRoot);

I have also added some extra things like a which and a golangci-lint --version command to validate the tool instalation, and some usage documentation in the README.md.

Any feedback is much appreciated.

[GusAntoniassi]

Fixes #1