Ship the Python README as the PyPI description

The `vtracer` package never declared a `readme`, so its PyPI page rendered
an empty description. Point `[project].readme` at the crate's README, and
mirror it in Cargo.toml alongside the core crate's convention.

Pin the documented install to 1.0.0a2 while the 1.0 line is pre-release:
`--pre` would resolve it, but a bare `pip install vtracer` still lands on
0.6.15, whose cp314 wheels segfault on any keyword argument (pyo3 0.19
predates CPython 3.14 moving `PyTupleObject::ob_item`, #124).

README.md

@@ -206,7 +206,7 @@ See [docs.rs/vtracer](https://docs.rs/vtracer/1.0.0-alpha.2/vtracer/) for the fu
 [`vtracer`](https://pypi.org/project/vtracer/) is also packaged as a Python native extension.
 
 ```sh
-pip install --pre vtracer
+pip install vtracer==1.0.0a2
 ```
 
 ```python