Add documentation for tagged enums with Serde

[Kriskras99]

As discussed in #586 here is some documentation for working with tagged enums.
I changed my implementation to a macro because it became tedious very fast (I need to deserialize an enum with 70 variants).

[Mingun]

Looks good! Can you also add a changelog entry

I also try to play with it in next days, because right now doctests are definitely will fail, because macro and usage in different blocks. We can merge both blocks or move macro implementation to it's own file and include! it in both blocks

[Kriskras99]

I've added a changelog entry and merged the two blocks. I've also included all your feedback except for the about using MapAccessDeserializer

[codecov-commenter]

Codecov Report

Merging #594 (858118e) into master (88a2f27) will increase coverage by 0.04%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
+ Coverage   64.43%   64.48%   +0.04%     
==========================================
  Files          36       36              
  Lines       16919    16921       +2     
==========================================
+ Hits        10902    10911       +9     
+ Misses       6017     6010       -7     

Flag	Coverage Δ
unittests	64.48% <0.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/mod.rs	69.28% <0.00%> (+0.26%)	⬆️
src/serde_helpers.rs	0.00% <ø> (ø)

... and 1 file with indirect coverage changes

[Mingun]

Link should be actually added:

Suggested change

	- [#594]: Add documentation for using tagged enums with Serde.
	- [#594]: Add documentation for using tagged enums with Serde.
	[#594]: https://github.com/tafia/quick-xml/pull/594

Also check cargo fmt -- it seems that there are some trailing spaces!

[Mingun]

I played with the implementation a bit. The reason why you've failed with MapAccessDeserializer in that that you have incorrect expectations of how the XML should be deserialized.

The types

#[derive(Deserialize, Debug, PartialEq)]
pub struct Root {
    pub one: TaggedEnum,
    pub two: TaggedEnum,
}

#[derive(Deserialize, Debug, PartialEq)]
#[serde(tag = "@tag")]
pub enum TaggedEnum {
    Spam(Spam),
    Ham,
}

#[derive(Deserialize, Debug, PartialEq)]
pub struct Spam {
    #[serde(rename = "@ham")]
    pub ham: u64,
}

describes the following XML

<root>
    <one tag="spam" ham="42" />
    <two tag="ham" />
</root>

you can make sure if you mentally replace each enumeration variant with an equivalent type: see example.

I was able to deserialize it successfully using approach with MapAccessDeserializer.

[Mingun]

I decided that since we now have a special module for helpers, add a macro there. That simple macro should help with typical tasks.

@dralley, @Kriskras99, if you have time could you please check grammar?

[Kriskras99]

@Mingun sorry I let this pull request sit. I have reviewed the documentation and did find some grammar issues, I'll open a PR to fix them!