Note Java's incompatibility with other specs

As noted in #87, the Java implementation doesn't match existing and upcoming specs for floating point output that require the shortest output. The implementation here attempts to follow Double.toString, which requires at least two digits of output.

README.md

@@ -8,6 +8,11 @@ The Java implementations are RyuFloat and RyuDouble under src/main/java/. The
 C implementation is in the ryu/ directory. Both cover 32 and 64-bit floating
 point numbers.
 
+*Note*: The Java implementation attempts to follow the Java specification for
+Double.toString [1], which requires outputting at least two digits. Other
+specifications, such as for JavaScript, require the shortest output. We may
+change the Java implementation in the future to support both.
+
 There is an experimental C low-level API and 128-bit implementation in ryu/.
 These are still subject to change.
 
@@ -29,6 +34,8 @@ Other implementations:
 | Factor           | Alexander Iljin    | https://github.com/AlexIljin/ryu              |
 | Go               | Caleb Spare        | https://github.com/cespare/ryu                |
 
+[1] https://docs.oracle.com/javase/10/docs/api/java/lang/Double.html#toString(double)
+
 ## Building, Testing, Running
 
 We use the Bazel build system (https://bazel.build). We recommend using the