Custom functions description

Functions.md

@@ -1,3 +1,7 @@
+* [How to use functions](#how-to-use-functions)
+* [String functions](#string-functions)
+* [Aggregating functions](#aggregating-functions)
+* [Custom functions](#custom-functions)
 
 ## How to use functions
 In cases when you want to retrieve function result from a query you have to declare function as a variable like:
@@ -36,3 +40,31 @@ val (min, max, avg) = FooTable.slice(minId, maxId, averageId).selecAll().map {
 }
 
 ```
+
+## Custom functions
+If you can't find your most loved function used in your database (as Exposed provides only basic support for classic SQL functions) you can define your own functions.
+
+Since Exposed 0.15.1 there multiple options to define custom functions:
+1. Function without parameters:
+```kotlin
+val sqrt = FooTable.id.function("SQRT")
+```
+In SQL representation it will be `SQRT(FooTable.id)`
+
+2. Function with additional parameters:
+```kotlin
+val replacedName = CustomFunction<String?>("REPLACE", VarCharColumnType(), FooTable.name, stringParam("foo"), stringParam("bar"))
+
+``` 
+`CustomFunction` class accept function name as a first parameter and resulting column type as second, after that you can provide any amount of parameters with will be separated by a comma.
+
+There are also shortcuts for string, long and datetime functions:
+* `CustomStringFunction`
+* `CustomLongFunction`
+* `CustomDateTimeFunction`
+
+The code above could be simplified to:
+```kotlin
+val replacedName = CustomStringFunction("REPLACE", FooTable.name, stringParam("foo"), stringParam("bar"))
+
+```