feat: add docs for read-after-write-consistency

doc/toturial/04-Read-After-Write-Consistency.md

@@ -0,0 +1,35 @@
+# 
+
+The goal of this tutorial is to explain how to enable the Read-After-Write-Consistency feature in WeSQL-Scale, in order to ensure that after enabling read-write separation, stale data will not be read.
+
+# Setting through the set command.
+
+If you have already set up a cluster and want to enable Read-After-Write-Consistency, the simplest way is using the MySQL "set" command:
+
+```
+# session
+set session read_after_write_consistency='SESSION'
+
+# global
+set global read_after_write_consistency='SESSION'
+
+```
+
+Currently, WeSQL-Scale supports these consistency levels:
+
+| EVENTUAL | No guarantee of consistency. |
+| --- | --- |
+| SESSION | Within the same connection, ensure that subsequent read requests can read previous write operations. (recommend) |
+| INSTANCE | Within the same instance (VTGate), ensure that subsequent read requests can read previous write operations, even if the read and write requests are not initiated by the same connection. |
+| GLOBAL | Within the same WeSQL-Scale cluster, ensure that any read request can read previous write operations. |
+
+# Setting via launch parameters
+
+If you need to set the default value of read_write_splitting_policy, you can pass it as a startup parameter for the vtgate process:
+```bash
+vtgate \
+    # enable read-write-splitting and load balancing
+    --read_after_write_consistency session
+    # other necessary command line options
+    ...
+```