Add documentation for installing extension.

[AaronJan]

No description provided.

[thaJeztah]

Could you wrap these lines at 80 chars?

Also, should be mentioned that docker-php-ext-install doesn't handle dependencies and that this has to be done manually?

[AaronJan]

Good points, will do this tomorrow.

[tianon]

These changes should be made to php/content.md instead. The php/README.md file is generated from a number of other files.

[tianon]

See https://github.com/docker-library/docs/blob/master/README.md#what-are-all-these-files for more details.

[AaronJan]

@tianon Thank you for point that out.
Working like crazy recently, may be a couple days delay on this.

[AaronJan]

New pull request [https://github.com//pull/130]