enforce 16MB limit on predicate size (#80)

Signed-off-by: Brian DeHamer <[email protected]>

README.md

@@ -81,12 +81,14 @@ See [action.yml](action.yml)
     # URI identifying the type of the predicate.
     predicate-type:
 
-    # JSON string containing the value for the attestation predicate. Must
-    # supply exactly one of "predicate-path" or "predicate".
+    # String containing the value for the attestation predicate. String length
+    # cannot exceed 16MB. Must supply exactly one of "predicate-path" or
+    # "predicate".
     predicate:
 
-    # Path to the file which contains the JSON content for the attestation
-    # predicate. Must supply exactly one of "predicate-path" or "predicate".
+    # Path to the file which contains the content for the attestation predicate.
+    # File size cannot exceed 16MB. Must supply exactly one of "predicate-path"
+    # or "predicate".
     predicate-path:
 
     # Whether to push the attestation to the image registry. Requires that the
@@ -124,6 +126,11 @@ processed in batches 50. After the initial group of 50, each subsequent batch
 will incur an exponentially increasing amount of delay (capped at 1 minute of
 delay per batch) to avoid overwhelming the attestation API.
 
+### Predicate Limits
+
+Whether supplied via the `predicate` or `predicatePath` input, the predicate
+string cannot exceed 16MB.
+
 ## Examples
 
 ### Identify Subject by Path