Merge pull request #20 from davido:extend_readme_for_update_instruction

PiperOrigin-RevId: 274961218
Change-Id: I6ef304dd525ab15c8f5d3deea1422706475f9c4a

Author: copybara-github

README.md

@@ -6,6 +6,50 @@ The source code of the Bazel Tools for Java is currently checked in the [bazel
 repository](https://github.com/bazelbuild/bazel). The source code will be moved 
 incrementally to this repository.
 
+# Upgrade a Bazel project to use custom java_tools version
+
+To use a specific java_tools release in your Bazel project please add the `http_archive`
+definitions in your WORKSPACE file and set the options `--java_toolchain` and/or
+`--host_java_toolchain` accordingly. All java_tools releases can be found under
+https://github.com/bazelbuild/java_tools/releases.
+
+For example to use java_tools_javac11-v6.0 you can add the following to the WORKSPACE
+file:
+
+```
+http_archive(
+    name = "remote_java_tools_linux",
+    sha256 = "37acb8380b1dd6c31fd27a19bf3da821c9b02ee93c6163fce36f070a806516b5",
+    urls = [
+        "https://mirror.bazel.build/bazel_java_tools/releases/javac11/v6.0/java_tools_javac11_linux-v6.0.zip",
+        "https://github.com/bazelbuild/java_tools/releases/download/javac11-v6.0/java_tools_javac11_linux-v6.0.zip",
+    ],
+)
+http_archive(
+    name = "remote_java_tools_windows",
+    sha256 = "384e138ca58842ea563fb7efbe0cb9c5c381bd4de1f6a31f0256823325f81ccc",
+    urls = [
+        "https://mirror.bazel.build/bazel_java_tools/releases/javac11/v6.0/java_tools_javac11_windows-v6.0.zip",
+        "https://github.com/bazelbuild/java_tools/releases/download/javac11-v6.0/java_tools_javac11_windows-v6.0.zip",
+    ],
+)
+http_archive(
+    name = "remote_java_tools_darwin",
+    sha256 = "5a9f320c33424262e505151dd5c6903e36678a0f0bbdaae67bcf07f41d8c7cf3",
+    urls = [
+        "https://mirror.bazel.build/bazel_java_tools/releases/javac11/v6.0/java_tools_javac11_darwin-v6.0.zip",
+        "https://github.com/bazelbuild/java_tools/releases/download/javac11-v6.0/java_tools_javac11_darwin-v6.0.zip",
+    ],
+)
+```
+
+and set the command line options according to the OS:
+
+```
+  --java_toolchain=@remote_java_tools_linux//:toolchain \
+  --host_java_toolchain=@remote_java_tools_linux//:toolchain
+```
+
 # Releases
 
 The releases can be found under [java_tools/releases](https://github.com/bazelbuild/java_tools/releases)