Document a better module build command.

`m apps_only dist` avoids building a compressed APEX that adb cannot
install (cf. b/218158375#comment2), and it also puts the APEX in a more
convenient location.

Test: m apps-only dist
      m
  verify that the APEXes created both ways have the same md5sum
Bug: 218158375
Change-Id: Iee3fc9c4cf154215e844bf5fbbcb190bf6694d76
diff --git a/build/README.md b/build/README.md
index 6bb97c5..807d6d5 100644
--- a/build/README.md
+++ b/build/README.md
@@ -58,13 +58,13 @@
 3.  Build the module:
 
     ```
-    m
+    m apps_only dist
     ```
 
 4.  Install the module and reboot:
 
     ```
-    adb install out/target/product/module_<arch>/system/apex/com.android.art.apex
+    adb install out/dist/com.android.art.apex
     adb reboot
     ```