Add a README.md for async_safe

Particularly to document why both this and liblog exist, when they do
essentially the same thing.

Test: n/a
Change-Id: I216194402a12270cfbb6bc9b840d054dc9c1dc16

libc/async_safe/README.md

@@ -0,0 +1,10 @@
+# async_safe logging
+
+This library provides an async_safe implementation for formatting and writing log messages to logd.
+
+Note that the liblog implementation connects a single socket to logd and uses a RWLock to manage
+it among threads, whereas these functions connect to liblog for each log message.  While it's
+beneficial to have this lock-free and therefore async_safe mechanism to write to logd, connecting
+a socket for each message does not scale well under load.  It was also determined to be too
+costly to connect a socket for each thread as some processes, such as system_server, have over 100
+threads.  Therefore, we maintain these two separate mechanisms.