Add doc about linker.config.pb

Since there's no mention about linker.config.pb, one may get confused
they want to find the artifact from the device.

Especially, /system/etc/linker.config.pb is not just compiled into .pb,
but is amended during "make" to populated(or append) "provideLibs" key
at make phase.

Because /system/etc/linker.config.pb is a very crucial part of the
system, it deserves more explanation.

Bug: n/a
Test: documentation
Change-Id: Ib6db77a42ec842ab0ecea15a44f9557345b57a58

diff --git a/README.md b/README.md
index 118707f..d560c57 100644 (file)
--- a/README.md
+++ b/README.md
@@ -17,9 +17,17 @@ TODO: explain inputs (e.g. /system/etc/public.libraries.txt,
 
 Linker configuration file can be used to add extra information while linkerconfig
 creates linker configuration with the module. This module can be defined as
-'linker_config' from Soong, and it will be translated as protobuf file at build
+`linker_config` from Soong, and it will be translated as protobuf file at build
 time.
 
+A linker configuration file(linker.config.json) is compiled into a protobuf at build time
+by `conv_linker_config`. You can find the compiled file under `<base>/etc/linker.config.pb`.
+For example, `/apex/com.android.art/etc/linker.config.pb` is a configuration for the `com.android.art`
+APEX.
+
+`/system/etc/linker.config.pb`(or its source module `system_linker_config`) is special because
+its `provideLibs` key is generated at build time.
+
 #### Format
 
 linker.config.json file is in json format which can contain properties as below.