readme: document ./build -s

Author: cirosantilli

README.adoc

@@ -5195,9 +5195,13 @@ if you make any changes to that package after the initial build: <<rebuild>>
 
 If you are comparing two versions of have a package that takes considerable time to build, one on each branch, you may want to keep two builds around to make things even faster.
 
-One simple approach is to simply use two checkouts of this repository.
+One simple approach is to simply use two checkouts of this repository, or to use the `-s` option:
 
-While <<ccache>> does help with this, it still does not necessarily make compilation instantaneous due to configuration file reading.
+....
+./build -s mybranch
+....
+
+While <<ccache>> does help to speed up the full rebuild, it still does not necessarily make compilation instantaneous due to configuration file reading.
 
 One alternative is to do:
 

build-usage.adoc

@@ -28,6 +28,10 @@
 |`-p` |                     |Pass extra arguments to the `rootfs_post_build_script`.
 |`-S` |                     |Don't build QEMU with SDL support.
                              Graphics such as X11 won't work, only the terminal.
+|`-s` |                     |Add a custom suffix to the build.
+                             E.g., doing `./build -s mysuf` puts all the build output
+                             into `out/x86_64-mysuf`. This allows keep multiple builds around
+                             when you checkout between branches.
 |`-v` |                     |Do a verbose build.
 |===