Update README to be consistent with core Protovalidate repo (#330)

* Updates this implementation's README to incorporate changes from this
weekend's update Protovalidate's README. Includes correct field number
for `last_name`.
* Additionally fixes one incorrectly capitalized title.

Author: jrinehart-buf

README.md

@@ -1,38 +1,33 @@
-[![The Buf logo](.github/buf-logo.svg)][buf] 
+[![The Buf logo](.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)
 
-[Protovalidate][protovalidate] provides standard annotations to validate common constraints on messages and fields, as well as the ability to use [CEL][cel] to write custom constraints. It's the next generation of [protoc-gen-validate][protoc-gen-validate], the only widely used validation library for Protobuf.
+[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].
 
 With Protovalidate, you can annotate your Protobuf messages with both standard and custom validation rules:
 
 ```protobuf
 syntax = "proto3";
 
-package banking.v1;
+package acme.user.v1;
 
 import "buf/validate/validate.proto";
 
-message MoneyTransfer {
-  string to_account_id = 1 [
-    // Standard rule: `to_account_id` must be a UUID
-    (buf.validate.field).string.uuid = true
-  ];
+message User {
+  string id = 1 [(buf.validate.field).string.uuid = true];
+  uint32 age = 2 [(buf.validate.field).uint32.lte = 150]; // We can only hope.
+  string email = 3 [(buf.validate.field).string.email = true];
+  string first_name = 4 [(buf.validate.field).string.max_len = 64];
+  string last_name = 5 [(buf.validate.field).string.max_len = 64];
 
-  string from_account_id = 2 [
-    // Standard rule: `from_account_id` must be a UUID
-    (buf.validate.field).string.uuid = true
-  ];
-
-  // Custom rule: `to_account_id` and `from_account_id` can't be the same.
   option (buf.validate.message).cel = {
-    id: "to_account_id.not.from_account_id"
-    message: "to_account_id and from_account_id should not be the same value"
-    expression: "this.to_account_id != this.from_account_id"
+    id: "first_name_requires_last_name"
+    message: "last_name must be present if first_name is present"
+    expression: "!has(this.first_name) || has(this.last_name)"
   };
 }
 ```
@@ -67,7 +62,7 @@ Highlights for Python developers include:
 * A comprehensive RPC quickstart for [Python and gRPC][grpc-python]
 * A [migration guide for protoc-gen-validate][migration-guide] users
 
-## Additional Languages and Repositories
+## Additional languages and repositories
 
 Protovalidate isn't just for Python! You might be interested in sibling repositories for other languages: 
 
@@ -81,18 +76,13 @@ Additionally, [protovalidate's core repository](https://github.com/bufbuild/prot
 - [Protovalidate's Protobuf API][validate-proto]
 - [Conformance testing utilities][conformance] for acceptance testing of `protovalidate` implementations
 
-## Contribution
+## Contributing
 
 We genuinely appreciate any help! If you'd like to contribute, check out these resources:
 
 - [Contributing Guidelines][contributing]: Guidelines to make your contribution process straightforward and meaningful
 - [Conformance testing utilities](https://github.com/bufbuild/protovalidate/tree/main/docs/conformance.md): Utilities providing acceptance testing of `protovalidate` implementations
 
-## Related Sites
-
-- [Buf][buf]: Enterprise-grade Kafka and gRPC for the modern age
-- [Common Expression Language (CEL)][cel]: The open-source technology at the core of Protovalidate
-
 ## Legal
 
 Offered under the [Apache 2 license][license].