Add useful references to the "keys" node.

Author: fdb

node/reference/data/import_csv.md

@@ -11,3 +11,7 @@ Import a comma-separated value file containing tabular data.
 * **Delimiter**: The character used to separate columns.
 * **Quotes**: The type of quotes used in the file.
 * **Number Separator**: The character used to separate the fractional part.
+
+## Notes
+
+If you want to retrieve the headers of a CSV file, use the [keys](/node/reference/list/keys.html) node.

node/reference/data/lookup.md

@@ -9,3 +9,8 @@ Look up a value in a table or object.
 
 * **List**: The input value.
 * **Key**: The key to lookup.
+
+
+## Notes
+* This node is often used after a [import CSV](/node/reference/data/import_csv.html) node to retrieve the values of specific columns.
+* An advanced feature is to use it to retrieve the properties of graphics objects, like the width of a piece of text. For that, type `bounds.width` in the key field. Use the [keys](/node/reference/list/keys.html) node to discover all possible properties of objects.