Document that binding needs also the type Procfile

[c0d1ngm0nk3y]

It is only documented that the binding needs to have the key Procfile, but in fact it also needs the type to be Procfile.

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[dmikusa]

This buildpack will participate if one or all of the following conditions are met:

The application contains a Procfile
A Binding exists with type Procfile and secret containing a Procfile
The BP_PROCFILE_DEFAULT_PROCESS environment variable is set to a non-empty value

We typically document the required type in the section that describes the condition for participation, since it's usually the type that is required there.

I'm not opposed to adding it to the table as well, but it will leave the tables inconsistent across buildpacks. My only ask would be that as you're submitting PRs for other buildpacks, if you remember, update the bindings table in the README for those as well. I'll try to remember to do the same.