minadbd: Update README.md.

Test: N/A
Change-Id: I7f4648f2b357de7d9700e38552a0156d13c1bd0b

minadbd/README.md

@@ -1,8 +1,24 @@
-minadbd is now mostly built from libadbd. The fuse features are unique to
-minadbd, and services.c has been modified as follows:
+minadbd
+=======
 
-  - all services removed
-  - all host mode support removed
-  - `sideload_service()` added; this is the only service supported. It
-    receives a single blob of data, writes it to a fixed filename, and
-    makes the process exit.
+`minadbd` is analogous to the regular `adbd`, but providing the minimal services to support
+recovery-specific use cases. Generally speaking, `adbd` = `libadbd` + `libadbd_services`, whereas
+`minadbd` = `libadbd` + `libminadbd_services`.
+
+Although both modules may be installed into the recovery image, only one of them, or none, can be
+active at any given time.
+
+- The start / stop of `adbd` is managed via system property `sys.usb.config`, when setting to `adb`
+  or `none` respectively. Upon starting recovery mode, `adbd` is started in debuggable builds by
+  default; otherwise `adbd` will stay off at all times in user builds. See the triggers in
+  `bootable/recovery/etc/init.rc`.
+
+- `minadbd` is started by `recovery` as needed.
+  - When requested to start `minadbd`, `recovery` stops `adbd` first, if it's running; it then forks
+    and execs `minadbd` in a separate process.
+  - `minadbd` talks to host-side `adb` server to get user requests.
+    - `minadbd` handles some requests directly, e.g. querying device properties for rescue service.
+    - `minadbd` communicates with `recovery` to fulfill requests regarding package installation. See
+      the comments in `bootable/recovery/install/adb_install.cpp` for the IPC protocol between
+      `recovery` and `minadbd`.
+  - Upon exiting `minadbd`, `recovery` restarts `adbd` if it was previously running.