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/api.py b/ethosu/vela/api.py
index 22f81d0..e628600 100644
--- a/ethosu/vela/api.py
+++ b/ethosu/vela/api.py
@@ -25,9 +25,9 @@
 
 import numpy
 
-API_version_major = 1
-API_version_minor = 0
-api_version = f"{API_version_major}.{API_version_minor}"
+API_VERSION_MAJOR = 1
+API_VERSION_MINOR = 0
+API_VERSION = f"{API_VERSION_MAJOR}.{API_VERSION_MINOR}"
 
 
 class NpuAccelerator(Enum):
@@ -388,13 +388,13 @@
         self.rescale: Optional[Tuple] = None
 
 
-def npu_get_API_version():
+def npu_get_api_version():
     """
     Public facing API to get the API version
     :return: int, the 16 most significant bits, corresponding to major version
             the 16 least significant bits, corresponding to minor version
     """
-    version = (API_version_major << 16) | (API_version_minor & 0xFFFF)
+    version = (API_VERSION_MAJOR << 16) | (API_VERSION_MINOR & 0xFFFF)
     return version