Update README.md

cereal/README.md

@@ -29,6 +29,50 @@ then means breaking backwards-compatibility with all old logs of your fork. So w
 [custom.capnp](custom.capnp) that we will leave empty in mainline cereal/openpilot. **If you only modify those, you can ensure your
 fork will remain backwards-compatible with all versions of mainline openpilot and your fork.**
 
+An example of compatible changes:
+```diff
+diff --git a/cereal/custom.capnp b/cereal/custom.capnp
+index 3348e859e..3365c7b98 100644
+--- a/cereal/custom.capnp
++++ b/cereal/custom.capnp
+@@ -10,7 +10,11 @@ $Cxx.namespace("cereal");
+ # DO rename the structs
+ # DON'T change the identifier (e.g. @0x81c2f05a394cf4af)
+
+-struct CustomReserved0 @0x81c2f05a394cf4af {
++struct SteeringInfo @0x81c2f05a394cf4af {
++  active @0 :Bool;
++  steeringAngleDeg @1 :Float32;
++  steeringRateDeg @2 :Float32;
++  steeringAccelDeg @3 :Float32;
+ }
+
+ struct CustomReserved1 @0xaedffd8f31e7b55d {
+diff --git a/cereal/log.capnp b/cereal/log.capnp
+index 1209f3fd9..b189f58b6 100644
+--- a/cereal/log.capnp
++++ b/cereal/log.capnp
+@@ -2558,14 +2558,14 @@ struct Event {
+
+     # DO change the name of the field
+     # DON'T change anything after the "@"
+-    customReservedRawData0 @124 :Data;
++    rawCanData @124 :Data;
+     customReservedRawData1 @125 :Data;
+     customReservedRawData2 @126 :Data;
+
+     # DO change the name of the field and struct
+     # DON'T change the ID (e.g. @107)
+     # DON'T change which struct it points to
+-    customReserved0 @107 :Custom.CustomReserved0;
++    steeringInfo @107 :Custom.SteeringInfo;
+     customReserved1 @108 :Custom.CustomReserved1;
+     customReserved2 @109 :Custom.CustomReserved2;
+     customReserved3 @110 :Custom.CustomReserved3;
+```
+
+---
+
 Example
 ---
 ```python