doc: add missing jsondoc.go from e093206a2a

Change-Id: I9f91a330f854e47888bd5f6286e8da4fecde262f

doc/protocol/jsonsign.md

@@ -0,0 +1,42 @@
+# JSON signing & verification
+
+A Perkeep server will typically expose a JSON signing handler. The operations for the signing handler are available at URL paths that are under the [Discovery protocol](discovery) response's **jsonSignRoot** value.
+
+The three handlers paths are:
+
+* **[jsonSignRoot]**/camli/sig/discovery
+* **[jsonSignRoot]**/camli/sig/sign
+* **[jsonSignRoot]**/camli/sig/verify
+
+## Discovery
+
+The discovery handler, in response to a GET request with no options,
+returns a
+[SignDiscovery](https://perkeep.org/pkg/types/camtypes/#SignDiscovery)
+value, such as:
+
+```
+{
+    "publicKey": "/sighelper/camli/sha1-f72d9090b61b70ee6501cceacc9d81a0801d32f6",
+    "publicKeyBlobRef": "sha1-f72d9090b61b70ee6501cceacc9d81a0801d32f6",
+    "publicKeyId": "94DE83C46401800C",
+    "signHandler": "/sighelper/camli/sig/sign",
+    "verifyHandler": "/sighelper/camli/sig/verify"
+}
+```
+
+## Signing
+
+The signing handler requires a POST request (of either
+type `application/x-www-form-urlencoded` or `multipart/form-data`) and accepts
+parameters:
+
+* **json**: the unsigned JSON to sign
+
+## Verification
+
+The verification handler requires a POST request (of either
+type `application/x-www-form-urlencoded` or `multipart/form-data`) and accepts
+parameters:
+
+* **sjson**: the signed JSON to verify