[docs] moved into folders

docs/user/customize/docs.user.customize.custom_field.md

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+---
+{
+	"_label": "Custom Field"
+}
+---
+A very common customization is adding of custom fields. You can add Custom Fields in any Master or Transaction in ERPNext. To add a Custom Field, go to:
+
+> Setup > Custom Field > New Custom Field
+
+In the form:
+
+- Select the Document on which you want to add the Custom Field.
+- Select the Type of field and the Options (see section on field types).
+- Select where you want the field to appear in the Form (“after field” section).
+
+and save the Custom Field. When you open a new / existing form of the type you selected in step 1, you will see it with the Custom Fields.
+
+#### Naming
+
+Many times you want your fields to be carried over from one form to another. For example, you may have added a Custom Field in Quotation that you want to include in Sales Order when a Sales Order is created from the Quotation. This is simple in ERPNext, just make sure the fields have the same “fieldname”