Doc: list all APIs equivalent between gomonkey and xgo

[xhd2015]

Since we are helping open projects migrating from gomonkey to xgo, see #18.
We need to draft a migration document.

[xhd2015]

Dot import : . "github.com/agiledragon/gomonkey/v2"
Can be removed

[xhd2015]

Patch reset:

p := gomonkey.NewPatches()
defer p.Reset()

Migration: Can be removed.

This is usually a clean up. Since xgo isolates mocks among different goroutines, and automatically clears when goroutine exit, there is no need to do this.

[xhd2015]

ApplyFuncReturn:

p = p.ApplyFuncReturn(prometheustimer.New, nil, errors.New(t.Name()))

Can be rewritten as:

mock.Mock(prometheustimer.New, func(ctx context.Context, fn *core.FuncInfo, args, results core.Object) error {
	return errors.New(t.Name())
})

Though seems longer, but the signature will not change. it's more obvious.