Add mutation example (write query)

rfc/0003 RFC GraphQL API for Duniter Client.md

@@ -267,11 +267,20 @@ You can use these to paginate your list and avoid a "timebomb" request.
 leading to slower and bigger responses that can, at the end, crash the server.*
 
 ```javascript
+
+# Query with variables and default values
 query getIdentities($offset: Int = 0, $limit: Int = 1000) {
     Identities {
         uid
     }
 }
+
+# Variables values for the request
+{
+    "offset": 1000,
+     "limit": 1000
+}
+
 ```
 
 List response:
@@ -292,4 +301,30 @@ List response:
 
 **Send an Identity Document:**
 
+GraphQL use another type of query to modifiy server's data. It is called "mutation".
+As arguments, you must use "input objects" defined in the schema on the server.
+
+```javascript
+
+# Send a write mutation
+mutation sendIdentityDocument($identityRaw: InputIdentityRaw!) {
+    Identity
+}
+
+# Variables
+{
+  "identityRaw": "Version: 10\n
+        Type: Identity\n
+        Currency: CURRENCY_NAME\n
+        Issuer: PUBLIC_KEY\n
+        UniqueID: USER_ID\n
+        Timestamp: BLOCK_UID\n
+        SIGNATURE"
+}
+```
+
+## Duniter Server module
+
+The Duniter module handling the GraphQL API on the server.
+
 TODO