Add info about using bootstrap property to the guide

[samdark]

Probably the best place is http://www.yiiframework.com/doc-2.0/guide-configuration.html

[samdark]

Re-thought about it. That's a bit too advanced feature to be covered in the guide. phpdoc describes it well. A full featured tutorial fits cookbook better.

[qiangxue]

It's already described in structure-applications.md and is planned to be covered in more details in runtime-bootstrapping.md.

[samdark]

Haven't noticed it. What is planned for runtime-bootstrapping.md? Examples?

[qiangxue]

runtime-*.md will describe how a request is being handled from the entry script till the output. It's the dynamic aspect of the framework, while structure-*.md describes the static structures of the framework.

I'm closing this issue since we already have it tracked in README.md.