update SPEC in light of comment commit changes

---
type: change
message: update SPEC in light of comment commit changes
change_hash: AEy+i6N0Rz2qtwfADoz55EAMO/rpQfCg6hPx9UTZGi6v
credentials:
- type: pgp_signature
  pub_key_id: 95C46FA6A41148AC
  body: 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
  account: mediocregopher

SPEC.md

@@ -64,7 +64,7 @@ access_controls:
 
 When a change commit (see Commits section) is being signed by a signifier there
 is an expected data format for the data to be signed. The format is a SHA-256
-hash of the following pieces of data concatenated together (the change hash):
+hash of the following pieces of data concatenated together:
 
 * A uvarint indicating the number of bytes in the commit message.
 * The message.
@@ -86,6 +86,19 @@ hash of the following pieces of data concatenated together (the change hash):
 The raw output from the SHA-256 is then prepended with a `0` byte (for forward
 compatibility). The result is the raw change hash.
 
+# Comment Message Hash
+
+When a comment commit (see Commits section) is being signed by the signifier of
+the author there is an expected data format for the data to be signed, very
+similar to how change hashes are signed. The format is a SHA-256 hash of the
+following pieces of data communicated together:
+
+* A uvarint indicating the number of bytes in the comment message.
+* The message.
+
+The raw output from the SHA-256 is then prepended with a `0` byte (for forward
+compatibility). The result is the raw comment hash.
+
 # Credentials
 
 All file changes need to have some kind of credential to be accepted into the
@@ -180,6 +193,7 @@ Example credential commit message:
 some_user_id pgp sig of commits AAA..BBB with key CCC
 
 ---
+type: credential
 credentialed_hash: XXX
 credentials:
     - type: pgp_signature
@@ -188,6 +202,37 @@ credentials:
       body: "base-64 signature body"
 ```
 
+## Comment Commits
+
+Commits of type `comment` contain a message for others to read. The commit
+message head is not spec'd, but should be a human-readable description of "who
+is commenting what".
+
+Example credential commit message:
+
+```
+some_user_id has commented: Hey all, how's it going?
+
+---
+type: comment
+
+# The message_hash is computed from the message, and reproduced here for
+# forwards compatibility. See the Comment Message Hash section.
+message_hash: XXX
+message: >
+    Heay all, how's it going?
+
+    Just wanted to pop by and say howdy.
+
+# credentials can contain a signature from the author of this comment's
+# message_hash.
+credentials:
+    - type: pgp_signature
+      account_id: some_user_id
+      pub_key_id: CCC
+      body: "base-64 signature body"
+```
+
 # Branches
 
 dehub branches correspond 1-to-1 with branches in the underlying git repo. All