add link of contributor guide for new functions to src (#7240)

datafusion/expr/src/aggregate_function.rs

@@ -25,6 +25,8 @@ use std::{fmt, str::FromStr};
 use strum_macros::EnumIter;
 
 /// Enum of all built-in aggregate functions
+// Contributor's guide for adding new aggregate functions
+// https://arrow.apache.org/datafusion/contributor-guide/index.html#how-to-add-a-new-aggregate-function
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Hash, EnumIter)]
 pub enum AggregateFunction {
     /// count

datafusion/expr/src/built_in_function.rs

@@ -34,6 +34,8 @@ use strum_macros::EnumIter;
 use lazy_static::lazy_static;
 
 /// Enum of all built-in scalar functions
+// Contributor's guide for adding new scalar functions
+// https://arrow.apache.org/datafusion/contributor-guide/index.html#how-to-add-a-new-scalar-function
 #[derive(Debug, Clone, PartialEq, Eq, Hash, EnumIter, Copy)]
 pub enum BuiltinScalarFunction {
     // math functions

docs/source/contributor-guide/index.md

@@ -221,9 +221,11 @@ Below is a checklist of what you need to do to add a new scalar function to Data
   - a new line in `signature` with the signature of the function (number and types of its arguments)
   - a new line in `create_physical_expr`/`create_physical_fun` mapping the built-in to the implementation
   - tests to the function.
-- In [core/tests/sql](../../../datafusion/core/tests/sql), add a new test where the function is called through SQL against well known data and returns the expected result.
+- In [core/tests/sqllogictests/test_files](../../../datafusion/core/tests/sqllogictests/test_files), add new `sqllogictest` integration tests where the function is called through SQL against well known data and returns the expected result.
+  - Documentation for `sqllogictest` [here](../../../datafusion/core/tests/sqllogictests/README.md)
 - In [expr/src/expr_fn.rs](../../../datafusion/expr/src/expr_fn.rs), add:
   - a new entry of the `unary_scalar_expr!` macro for the new function.
+- Add SQL reference documentation [here](../../../docs/source/user-guide/sql/scalar_functions.md)
 
 ### How to add a new aggregate function
 
@@ -241,7 +243,9 @@ Below is a checklist of what you need to do to add a new aggregate function to D
   - a new line in `signature` with the signature of the function (number and types of its arguments)
   - a new line in `create_aggregate_expr` mapping the built-in to the implementation
   - tests to the function.
-- In [tests/sql](../../../datafusion/core/tests/sql), add a new test where the function is called through SQL against well known data and returns the expected result.
+- In [core/tests/sqllogictests/test_files](../../../datafusion/core/tests/sqllogictests/test_files), add new `sqllogictest` integration tests where the function is called through SQL against well known data and returns the expected result.
+  - Documentation for `sqllogictest` [here](../../../datafusion/core/tests/sqllogictests/README.md)
+- Add SQL reference documentation [here](../../../docs/source/user-guide/sql/aggregate_functions.md)
 
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