Add Gradle tasks to generate protobuf, update readme

generators/build.gradle.kts

@@ -62,10 +62,13 @@ projectTest {
     workingDir = rootDir
 }
 
-val generateTests by task<JavaExec> {
+fun generator(fqName: String) = task<JavaExec> {
     classpath = the<JavaPluginConvention>().sourceSets["test"].runtimeClasspath
-
-    main = "org.jetbrains.kotlin.generators.tests.GenerateTestsKt"
-
+    main = fqName
     workingDir = rootDir
-}
+}
+
+val generateTests by generator("org.jetbrains.kotlin.generators.tests.GenerateTestsKt")
+
+val generateProtoBuf by generator("org.jetbrains.kotlin.generators.protobuf.GenerateProtoBufKt")
+val generateProtoBufCompare by generator("org.jetbrains.kotlin.generators.protobuf.GenerateProtoBufCompare")

generators/src/org/jetbrains/kotlin/generators/protobuf/GenerateProtoBuf.kt

@@ -27,7 +27,8 @@ import java.util.regex.Pattern
 //
 // * Windows: download and unpack from here: https://code.google.com/p/protobuf/downloads/list
 // * Ubuntu: install "protobuf-compiler" package
-// * Mac OS: install "protobuf" package from Homebrew or "protobuf-cpp" from MacPorts
+// * macOS:
+//     brew install https://raw.githubusercontent.com/udalov/protobuf261/master/protobuf261.rb
 // * You can also download source and build it yourself (https://code.google.com/p/protobuf/downloads/list)
 //
 // You may need to provide custom path to protoc executable, just modify this constant: