[Feature] Mention ThreeTenABP and android-retrostreams in README.

Author: zhanghai

README.md

@@ -10,14 +10,16 @@ dependencies {
 
 ## Usage
 
-This backported API is under `java8.nio.file`.
+The backported API is under `java8.nio.file`.
 
 No default `FileSystemProvider` implementation is bundled within this library, and the API is modified to allow dynamic install of providers.
 
 Before using the API, you need to set a default provider implementation with `FileSystemProvider.installDefaultProvider()`. More providers can be installed with `FileSystemProvider.installProvider()` at any time.
 
 Similarly, there is no default `FileTypeDetector` implementation, and you can install one with `Files.installFileTypeDetector()` at any time.
 
+This backport uses [android-retrostreams](https://github.com/retrostreams/android-retrostreams) for `java.util.stream` and [ThreeTenABP](https://github.com/JakeWharton/ThreeTenABP) for `java.time`, and they are exposed as an API dependency of this library.
+
 ## License
 
 [GNU General Public License, version 2, with the Classpath Exception](https://openjdk.java.net/legal/gplv2+ce.html)