README: document pruning mechanism.

Users should know that an automatic step is executed to make the image
small, because this might break their application in (hopefully) rare
situations. Also teach them it is possible to extend the paths where
important binaries are, and also skip pruning entirely if needed.

README.md

@@ -51,8 +51,17 @@ services:
 
 By default, the IOC will be built with statically linked EPICS libraries. If
 you **need** to link them dynamically, you must define the build target as
-`dynamic-link`. This will increase the resulting image size, since unused
-dependencies will also be copied.
+`dynamic-link`.
+
+For non-static builds, an automatic pruning step is executed to make the IOC
+image size small by removing unused artifacts. This step preserves all ELF
+executables inside `RUNDIR` and the IOC repository, as well as their dependent
+EPICS modules based on the linkage information. Additional paths can be
+provided in `args` under `APP_DIRS` key to extend the list of where important
+applications and shared libraries are, including any `dlopen(3)`ed libraries.
+Pruning step can be skipped entirely by providing the `SKIP_PRUNE=1` argument,
+but this is highly discouraged as it will increase significantly the image
+size.
 
 The resulting image contains a standard IOC run script, `lnls-run`, which will
 be run inside `RUNDIR` and will launch the container's command under procServ,