document usage of #[serde(flatten)] more thoroughly

I got a cool bug report on a project of mine that uses the csv crate: https://github.com/LPGhatguy/rojo/issues/145

I deserialize files into a `Vec` of a struct with a shape like this:

```rust
#[derive(Debug, Deserialize)]
struct SomethingEntry {
name: String,

#[serde(flatten)]
values: HashMap,
}
```

The actual structure uses `#[serde(flatten)]` to capture extra columns since it's used in a localization format for a game engine.

#### What version of the `csv` crate are you using?

`1.0.5`

#### Briefly describe the question, bug or feature request.

Deserializing a field that uses `#[serde(flatten)]` on a `HashMap` fails if the value in a record looks like a number.

#### Include a complete program demonstrating a problem.

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8d7a007589ba375a79a55b1d825ffdb5

```rust
extern crate csv; // 1.0.5
extern crate serde_derive; // 1.0.88
extern crate serde; // 1.0.88

use std::collections::HashMap;
use serde_derive::Deserialize;

#[derive(Debug, Deserialize)]
struct SomethingEntry {
name: String,

#[serde(flatten)]
values: HashMap,
}

fn main() {
let source = r#"name,stat
name,Benjamin
maxHealth,300"#;

let mut rdr = csv::Reader::from_reader(source.as_bytes());
let records: Vec = rdr.deserialize()
.map(Result::unwrap)
.collect();

println!("{:?}", records);
}
```

#### What is the observed behavior of the code above?

The program panics, since csv returns an error:
```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error(Deserialize { pos: Some(Position { byte: 24, line: 3, record: 2 }), err: DeserializeError { field: None, kind: Message("invalid type: integer `300`, expected a string") } })', src/libcore/result.rs:997:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

#### What is the expected or desired behavior of the code above?

The first entry should be have a name of `name` and `values` set to `{"stat": "Benjamin"}`.

The second entry should have a name of `maxHealth` and `values` set to `{"stat": "300"}`.

Since the code works for most inputs, I don't expect it to start failing to parse just because the user put something into the spreadsheet that looks like a number.

Start by reproducing the linked Rust playground example and read the full issue thread, including the linked csv crate report. The payload names no documentation file or test; done means adding a clear explanation of #[serde(flatten)] with HashMap and the numeric-looking CSV value case, including the expected behavior.

Written by the indexing model from the issue text.