Recipe / Add Native Ads Express

[lgvalle]

Scope of the PR

Adds a recipe on how to create and integrate the newish Native Ads Express.

Reasons to exist

• Having a single guide to walk you through the process of creating and Advert and integrating it is more useful than checking separate documentation for each task.
• Google documentation is not very clear about advert sizes. At first sight it looks like the sizes you define when creating an Ad is the size you need to use later when integrating it.
  This cookbook tries to clarify that.

[lgvalle]

You can better preview this doc here: https://github.com/novoda/android-demos/blob/963b36fff1344c09700db9e303724735c563be2c/Cookbook/add_native_express_adverts.md

[mr-archano]

LGTM, great job @lgvalle
One minor remark: you could have used the description to highlight how this recipe is more useful than looking at the official doc/sample (if any), eg:

• it's more concise
• it lists also the pre/post steps for X
• there's a quirk with Y that is not mentioned anywhere
• ...

[tasomaniac]

This still looks like 1. in the preview

[lgvalle]

Fixed here 74dcf57

[tasomaniac]

Put MobileAds into backticks maybe?

[lgvalle]

Fixed here 5f81b00

[tobiasheine]

Awesome, just one little remark:
Please add an entry including a link to the README.md

[lgvalle]

Updated here 3da83b8 @tobiasheine

[lgvalle]

You are right @mr-archano . I added a "Reasons to exist" block into the description. We can use the same approach in upcoming recipes. Is good to write down the reasons behind writing it