README: re-add desugaring block

People are never gonna figure it out otherwise.

This reverts commit 1701e4fde4.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

README.md

@@ -22,7 +22,18 @@ The tunnel library is [on Maven Central](https://search.maven.org/artifact/com.w
 implementation 'com.wireguard.android:tunnel:$wireguardTunnelVersion'
 ```
 
-The library makes use of Java 8 features, so be sure to support those in your gradle configuration with [desugaring](https://developer.android.com/studio/write/java8-support#library-desugaring)
+The library makes use of Java 8 features, so be sure to support those in your gradle configuration with [desugaring](https://developer.android.com/studio/write/java8-support#library-desugaring):
+
+```
+compileOptions {
+    sourceCompatibility JavaVersion.VERSION_1_8
+    targetCompatibility JavaVersion.VERSION_1_8
+    coreLibraryDesugaringEnabled = true
+}
+dependencies {
+    coreLibraryDesugaring "com.android.tools:desugar_jdk_libs:1.1.5"
+}
+```
 
 ## Translating