Issue 50 - Add documentation for PHP namespace options

[nigelhough]

This pull request directly relates to #50

This expands the documentation for PHP Code Generation specifically documenting functionality added in the following two pull requests to protocolbuffers/protobuf.

• Add new file option php_namespace. protobuf#3162
• Add file option php_metadata_namespace and ruby_package protobuf#4609

For testing and validating the documentation the following .proto files was used

MyMessage.proto

syntax = "proto3";

package foo.bar;

option php_namespace = "baz\\qux";

option php_metadata_namespace = "Foo";

message MyMessage {}

and the following compiler command

protoc --php_out=output MyMessage.proto

[Logofile]

I've got a SME reviewing this PR.