Add testlib readme.

Fixes #3453

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=272455172

Author: elharo

guava-testlib/README.md

@@ -0,0 +1,51 @@
+# Guava Testlib: Google Testing Libraries for Java
+
+Guava testlib is a set of Java classes for more convenient
+unit testing.
+
+## Adding Guava Testlib to your build
+
+Guava testlib's Maven group ID is `com.google.guava` and its artifact ID is `guava-testlib`.
+
+To add a dependency on Guava testlib using Maven, use the following:
+
+```xml
+<dependency>
+  <groupId>com.google.guava</groupId>
+  <artifactId>guava-testlib</artifactId>
+  <version>27.1-jre</version>
+  <scope>test</scope>
+</dependency>
+```
+
+To add a dependency using Gradle:
+
+```gradle
+dependencies {
+  test 'com.google.guava:guava-testlib:27.1-jre'
+}
+```
+
+## Links
+
+- [GitHub project](https://github.com/google/guava)
+- [Issue tracker: Report a defect or feature request](https://github.com/google/guava/issues/new)
+- [StackOverflow: Ask "how-to" and "why-didn't-it-work" questions](https://stackoverflow.com/questions/ask?tags=guava+java)
+- [guava-discuss: For open-ended questions and discussion](http://groups.google.com/group/guava-discuss)
+
+## IMPORTANT WARNINGS
+
+1. APIs marked with the `@Beta` annotation at the class or method level
+are subject to change. They can be modified in any way, or even
+removed, at any time. If your code is a library itself (i.e. it is
+used on the CLASSPATH of users outside your own control), you should
+not use beta APIs, unless you [repackage] them. **If your
+code is a library, we strongly recommend using the [Guava Beta Checker] to
+ensure that you do not use any `@Beta` APIs!**
+
+[Guava Beta Checker]: https://github.com/google/guava-beta-checker
+
+<!-- References -->
+
+[repackage]: https://github.com/google/guava/wiki/UseGuavaInYourBuild#what-if-i-want-to-use-beta-apis-from-a-library-that-people-use-as-a-dependency
+