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).

crates/vtracer-py/README.md

@@ -8,7 +8,7 @@ this crate adds image decoding and a Pythonic API.
 ## Install
 
 ```sh
-pip install vtracer
+pip install vtracer==1.0.0a2
 ```
 
 ## Usage