MLBEDSW-3424: Added API.md

- Added API.md that describes the external APIs.
- Renamed npu_get_api_version

Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>
Change-Id: I6e6e6103a889da656b4e00c3cce3eee60dfa844a
diff --git a/ethosu/vela/vela.py b/ethosu/vela/vela.py
index f03bae7..b93774d 100644
--- a/ethosu/vela/vela.py
+++ b/ethosu/vela/vela.py
@@ -30,7 +30,7 @@
 from . import stats_writer
 from . import tflite_writer
 from ._version import __version__
-from .api import api_version
+from .api import API_VERSION
 from .debug_database import DebugDatabase
 from .errors import InputFileError
 from .nn_graph import PassPlacement
@@ -192,7 +192,7 @@
     parser = argparse.ArgumentParser(prog="vela", description="Neural network model compiler for Arm Ethos-U NPUs")
     parser.add_argument("--version", action="version", version=__version__)
     parser.add_argument(
-        "--api-version", action="version", version=api_version, help="Displays the version of the external API."
+        "--api-version", action="version", version=API_VERSION, help="Displays the version of the external API."
     )
     parser.add_argument(
         "--supported-ops-report",