docs: add missing documentation for decimal datatype and protocol version 3

README.md

@@ -59,6 +59,7 @@ These protocol versions are supported over both HTTP and TCP.
 | ------- | ------------------------------------------------------------ | --------------------- |
 | **1**   | Over HTTP it's compatible with InfluxDB Line Protocol (ILP)  | All QuestDB versions  |
 | **2**   | 64-bit floats sent as binary, adds n-dimensional arrays      | 9.0.0+ (2025-07-11)   |
+| **3**   | adds decimals                                                | 9.2.0+ (2025-11-14)   |
 
 ## Getting Started
 

questdb-rs/src/ingress/mod.md

@@ -270,6 +270,21 @@ the configuration string: `protocol_version=2;`.
 
 **Note**: QuestDB server version 9.0.0 or later is required for array support.
 
+## Decimal Datatype
+
+The [`Buffer::column_dec`](Buffer::column_dec) method supports efficient ingestion of decimals using several convenient types:
+
+- floating-point representation with `&str`
+- decimals from the [`bigdecimal`](https://docs.rs/bigdecimal) crate
+- decimals from the [`rust_decimal`](https://docs.rs/rust_decimal) crate
+
+You must use protocol version 3 to ingest decimals. The HTTP transport will
+automatically enable it as long as you're connecting to an up-to-date QuestDB
+server (version 9.2.0 or later), but with TCP you must explicitly specify it in
+the configuration string: `protocol_version=3;`.
+
+**Note**: QuestDB server version 9.2.0 or later is required for decimal support.
+
 ## Timestamp Column Name
 
 The InfluxDB Line Protocol (ILP) does not give a name to the designated timestamp,