Writing DPG customizations documentation

[lmazuel]

As GA requirement for the project, we want to have explicitly written the documentation for service teams on how to grow-up their DPG into an improved SDK.

Documentation should be written with:

• Basics on customizations for this language
• One scenario, one section. Like a FAQ document

Required sections are (at the minimum):

• Change operation behavior (new signature, or any wrapping)
• Change client behavior (default pipeline configuration, etc.)

More sections might be added, and I'll update this initial text with date when I add them (for clarity)