Add README.

README.md

+## Onion Wrapper
+
+A Java library providing a wrapper for running a Tor client on Android, Unix and Windows. The
+wrapper can be used for running hidden services.
+
+The library supports the `obfs4`, `meek_lite` and `snowflake` pluggable transports.
+
+Binaries for Tor and pluggable transports are not included. They can be found in the following
+Maven artifacts: `org.briarproject:{tor,obfs4proxy,snowflake}-{android,linux,windows}`. The
+`obfs4proxy` artifact provides `obfs4` and `meek_lite`.
+
+On Android, the library uses
+[dont-kill-me-lib](https://code.briarproject.org/briar/dont-kill-me-lib) to hold a wake lock
+whenever Tor's network connection is enabled. The helper classes in `dont-kill-me-lib` can be used
+to work around certain manufacturer-specific power management restrictions that would prevent Tor
+from running in the background for long periods.