THRIFT-4750: as3 changes to build and publish to maven central
diff --git a/lib/as3/README.md b/lib/as3/README.md
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+# Apache Thrift ActionScript Library
+
+## Building
+
+We use gradle and gradlefx to build the as3 library. Unfortunately gradlefx requires
+an older version of gradle (2.5) but it still works - for now. If you use the docker
+container to do the build, the Adobe Flex SDK 4.6 is installed and the FLEX_HOME
+environment variable is configured:
+
+ dev@ubuntu:~/thrift$ docker run -v $(pwd):/thrift/src:rw -it thrift/thrift-build:ubuntu-bionic /bin/bash
+ root@7624b61bbf84:/thrift/src# cd lib/as3
+ root@7624b61bbf84:/thrift/src/lib/as3# ./gradlew -Prelease=true compileFlex
+
+ ...
+
+ :compileFlex UP-TO-DATE
+
+ BUILD SUCCESSFUL
+
+ Total time: 10.784 secs
+
+ root@7624b61bbf84:/thrift/src/lib/as3# ls -ls build/
+ total 4
+ 4 -rw-r--r-- 1 root root 1379 Jan 22 19:23 libthrift-as3.swc
+
+## Publishing
+
+We use a similar gradle-based signing and publishing mechanism as in the java
+library. See the java library [README.md](../java/README.md) for more details.
+
+To publish into a local .m2 repository you can mount a directory into the docker container,
+for example:
+
+ dev@ubuntu:~/thrift$ docker run -v~/.m2:/root/.m2 -v $(pwd):/thrift/src:rw -it thrift/thrift-build:ubuntu-bionic /bin/bash
+ root@7624b61bbf84:/thrift/src/lib/as3# ./gradlew -Prelease=true publishToMavenLocal
+
+You will find your `~/.m2` directory is now populated with a release build `swc`.