Update README (#344)

* Use absolute instead of relative link for Buf logo, so it displays
elsewhere (e.g. PyPI: https://pypi.org/project/protovalidate/)
* Use img.shields.io for version shield for consistency with other
projects

Author: stefanvanburen

README.md

@@ -1,10 +1,10 @@
-[![The Buf logo](.github/buf-logo.svg)][buf]
+[![The Buf logo](https://raw.githubusercontent.com/bufbuild/protovalidate/main/.github/buf-logo.svg)][buf]
 
 # protovalidate-python
 
 [![CI](https://github.com/bufbuild/protovalidate-python/actions/workflows/ci.yaml/badge.svg)](https://github.com/bufbuild/protovalidate-python/actions/workflows/ci.yaml)
 [![Conformance](https://github.com/bufbuild/protovalidate-python/actions/workflows/conformance.yaml/badge.svg)](https://github.com/bufbuild/protovalidate-python/actions/workflows/conformance.yaml)
-[![PyPI version](https://badge.fury.io/py/protovalidate.svg)](https://badge.fury.io/py/protovalidate)
+[![PyPI version](https://img.shields.io/pypi/v/protovalidate)](https://pypi.org/project/protovalidate)
 
 [Protovalidate][protovalidate] is the semantic validation library for Protobuf. It provides standard annotations to validate common rules on messages and fields, as well as the ability to use [CEL][cel] to write custom rules. It's the next generation of [protoc-gen-validate][protoc-gen-validate].