Improving plugin implementation documentation

[joelwurtz]

@dbu

i am still new to promises. lets integrate the answers of #30 (comment) into the documentation to make things more clear. i guess i am not the only one not familiar with promises, so having more details could help. maybe we should move the "write your own plugin" into its own chapter inside the plugins/ folder. in a way, its the least important, coming after using the existing plugins. it would be cool to show the core code of plugins to explain cases:

• alter a request
• alter a response
• prematurely end a request with a synthetic response (cache, vcr, mocking)
• start a new request and return that result instead
• abort a request prematurely with an error
• abort a response with an error (Error plugin, or for example server validation fails)

[dbu]

many of the plugins are now documented. but i think this is more ideas for the general plugin documentation, right?

[sagikazarmark]

I think this is about implementing a plugin: async first strategy, restarting the request, etc.

[dbu]

https://github.com/php-http/documentation/blob/master/plugins/build-your-own.rst