Add plugin documentation and examples

[abmyii]

Docs, as mentioned in #101.

I must admit that what I wrote was a bit rambling and incoherent and will need improvement and simplifying. I don't think I understand plugins properly! And as Einstein said:

“If you can't explain it to a six year old, you don't understand it yourself.”
(https://www.goodreads.com/quotes/19421-if-you-can-t-explain-it-to-a-six-year-old)

Haha!

I hope what I've written is workable and not totally useless, because it does seem so to me a bit unfortunately!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c1cc5fc). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #102   +/-   ##
=========================================
  Coverage          ?   94.64%           
=========================================
  Files             ?       10           
  Lines             ?      635           
  Branches          ?        0           
=========================================
  Hits              ?      601           
  Misses            ?       34           
  Partials          ?        0

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[howie6879]

Thanks for your great job

[abmyii]

Um, thanks! But don't you think it is too difficult to understand?

[abmyii]

Also, I noticed a little mistake - the last line is:

And you should a `developer.db` should be created.

But should be:

And `developer.db` should have been created.

[howie6879]

you can change at any time ~

https://docs.python-ruia.org/en/examples/spider_parent_class.html

[abmyii]

Shall I start a new PR?

[howie6879]

yep