orm: add missing 'nonull' attribute description (#15348)

Author: cstffx

vlib/orm/README.md

@@ -11,6 +11,7 @@
 - `[primary]` sets the field as the primary key
 - `[unique]` sets the field as unique
 - `[unique: 'foo']` adds the field to a unique group
+- `[nonull]` set the field as not null
 - `[skip]` field will be skipped
 - `[sql: type]` where `type` is a V type such as `int` or `f64`, or special type `serial`
 - `[sql: 'name']` sets a custom column name for the field
@@ -140,4 +141,4 @@ fn main() {
 }
 
 
-```
+```