Address review comments

Signed-off-by: Marcela Melara <[email protected]>

Author: marcelamelara

spec/predicates/README.md

@@ -12,11 +12,12 @@ community! Please see our [New Predicate Guidelines].
 This directory contains predicate specification types that have gone through
 our [vetting process], and may be of general interest:
 
--   [SLSA Provenance]: To describe the build process of a software artifact.
+-   [SLSA Provenance]: Describes how an artifact or set of artifacts was
+    produced.
 -   [Link]: For migration from [in-toto 0.9].
 -   [SCAI Report]: Evidence-based assertions about software artifact and
     supply chain attributes or behavior.
--   [Runtime Traces]: To capture runtime traces of software supply chain
+-   [Runtime Traces]: Captures runtime traces of software supply chain
     operations.
 -   [SLSA Verification Summary]: SLSA verification decision about a software
     artifact.

spec/v1.0/envelope.md

@@ -5,8 +5,19 @@ Version: [DSSE v1.0]
 The Envelope is the outermost layer of the attestation, handling
 authentication and serialization.
 
-The format and protocol are defined per [DSSE v1.0]. In the in-toto
-Attestation Framework, the DSSE payload is a base64-encoded JSON [Statement].
+## Schema
+
+The format and protocol are defined per [DSSE v1.0].
+
+## Fields
+
+The in-toto Attestation Framework has the following requirements for the
+standard DSSE fields.
+
+-   `payloadType` MUST be set to `application/vnd.in-toto+json`, which
+    indicates that the Envelope contains a JSON object with a `_type` field
+    specifying its schema.
+-   `payload` MUST be a base64-encoded JSON [Statement].
 
 [DSSE v1.0]: https://github.com/secure-systems-lab/dsse/blob/v1.0.0/envelope.md
 [Statement]: statement.md