Installing with PECL does not install `phptrace` CLI

[GuyPaddock]

The documentation mentions that PECL is a way to install this, but doesn't mention that if you install the extension through PECL you still need to:

1. Download the corresponding release from GitHub.
2. Unpack the tarball.
3. Run phpize from the extension/ folder to generate the Makefile.
4. Run make cli to build the CLI binary.
5. Install the CLI with sudo make install-cli.

In some respects, this defeats the purpose of using PECL for the install.

[michaeljoseph]

@GuyPaddock adding these instructions to the README would be step in the right direction at least- keen to PR these changes?