Add installation instructions to README

[andreasschroth]

I think you should add installation instructions to the README.

Not everyone might be aware that it's a PECL package that can easily be installed with pecl install.
I think for wider adoption proper installation instructions are important.

PS: Thank you for the package! Keep up the good work. 👍

[krakjoe]

Thanks for taking the time to open an issue :)

@remicollet can you add the normal install bits to the readme please ...