Add warnings that buf must be the first step.

Author: nedtwigg

plugin-gradle/README.md

@@ -524,6 +524,8 @@ black().pathToExe('C:/myuser/.pyenv/versions/3.8.0/scripts/black.exe')
 
 `com.diffplug.gradle.spotless.ProtobufExtension` [javadoc](https://javadoc.io/doc/com.diffplug.spotless/spotless-plugin-gradle/6.19.0/com/diffplug/gradle/spotless/ProtobufExtension.html), [code](https://github.com/diffplug/spotless/blob/main/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/ProtobufExtension.java)
 
+**WARNING** this step **must** be the first step in the chain, steps before it will be ignored. Thumbs up [this issue](https://github.com/bufbuild/buf/issues/1035) for a resolution, see [here](https://github.com/diffplug/spotless/pull/1208#discussion_r1264439669) for more details on the problem.
+
 ```gradle
 spotless {
   protobuf {

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/ProtobufExtension.java

@@ -66,6 +66,9 @@ public class BufFormatExtension {
 
 		BufFormatExtension(String version) {
 			this.step = BufStep.withVersion(version);
+			if (!steps.isEmpty()) {
+				throw new IllegalArgumentException("buf() must be the first step, move other steps after it. Thumbs up [this issue](https://github.com/bufbuild/buf/issues/1035) for a resolution, see [here](https://github.com/diffplug/spotless/pull/1208#discussion_r1264439669) for more details on the problem.");
+			}
 			addStep(createStep());
 		}