[Feature] Installation instructions using composer

[deiga]

Is your feature request related to a problem? Please describe.
I'd love to be able to move the installation of the extension to composer instead of inside our Dockerfile, so that we get automatic version upgrade notifications. But I wasn't able to figure out how to use the tracer through composer

Describe the solution you'd like
I'd like a section in the documentation explaining how to use the tracer library when installed through composer

[bwoebi]

Hey @deiga,

while we provide https://packagist.org/packages/datadog/dd-trace (which you are free to include if you want IDE autocompletion for example), composer packages can't provide extensions themselves, only dependencies on extensions, hence to install the tracer you'll eventually need the installer.

[deiga]

@bwoebi Oh, that makes sense! Could that be written explicitly in the docs so that other folks won't be confused as well? :)