Explain rationale behind PEP 517/518 improvements

[obestwalter]

In #1137 it got clear that it might be a good idea to extend the package docs a bit to explain why isolated_build needs a pyproject.toml. We could have done that more implicitly, but that would have been dirty and hidden away what it's all about to the packager, so I agree with the intention of @gaborbernat - we just need to explain it a bit clearer.

[ljluestc]

If you want to update the package documentation to provide clearer information about why isolated_build requires a pyproject.toml file, you can add a section like this:

## Using `isolated_build` with `pyproject.toml`

When using the `isolated_build` option in your `pyproject.toml` file, it allows your package to be built in an isolated environment using the PEP 517 build backend. This isolation ensures that your package's build dependencies are separate from your development environment, reducing the chance of conflicting dependencies.

To enable `isolated_build`, you need to have a valid `pyproject.toml` file in your project directory. The `pyproject.toml` file should define the necessary build backend configuration and dependencies for building your package.

Here's an example `pyproject.toml` configuration that utilizes `isolated_build`:

```toml
[build-system]
requires = ["setuptools >= 40.8.0", "wheel"]
build-backend = "setuptools.build_meta"
isolated_build = true

[tool.poetry.dependencies]
python = "^3.6"

By using isolated_build along with a properly configured pyproject.toml, you ensure a consistent and isolated build process for your package.

Feel free to adjust the content to match your project's documentation style and structure. This addition should provide clearer information to users about the purpose and benefits of using `isolated_build` with a `pyproject.toml` file.