psa: Document a test partition creation gotcha

The release script must be run from mbed-os root, otherwise the test
partition binaries will not be created. Add a note in the tool's README
to make this clear and hopefully help save someone some debugging time
in the future.

Author: Patater

tools/psa/README.md

@@ -51,6 +51,9 @@ optional arguments:
   -x ..., --extra ...  additional build parameters
 ```
 
+* The script must be run from the mbed-os folder via `tools/psa/release.py`,
+  otherwise the list of available tests will not be accurate and the test
+  partitions will not be properly generated.
 * When `MCU ` is not specified, the script compiles all the images for all the targets.
 * When `-t/--tc` is not specified, the script compiles with the default_toolchain speciified in targets.json.
 * When `-d/--debug` is not specified, the script compiles the images using the release profile.