Add customIdToUri Map to LicenseExpression

Fixes #380

Signed-off-by: Gary O'Neall <gary@sourceauditor.com>

model/SimpleLicensing/Classes/LicenseExpression.md

@@ -23,3 +23,6 @@ SPDX License Expressions provide a way for one to construct expressions that mor
   - type: xsd:string
   - minCount: 1
   - maxCount: 1
+- customIdToUri
+  - type: /Core/DictionaryEntry
+  - minCount: 0

model/SimpleLicensing/Properties/customIdToUri.md

@@ -0,0 +1,21 @@
+SPDX-License-Identifier: Community-Spec-1.0
+
+# customIdToUri
+
+## Summary
+
+Maps a LicenseRef or AdditionRef string for a Custom License or a Custom License Addition to its URI ID.
+
+## Description
+
+Within a License Expression, references can be made to a Custom License or a Custom License Addition.
+The License Expression syntax dictates any refence starting with a "LicenseRef-" or "AdditionRef-" refers to license or addition text not found in the SPDX list of licenses.
+These custom licenses must be a CustomLicense, a CustomLicenseAddtion, or a SimpleLicensingText which are identified with a unique URI identifier.
+The key for the DictionaryEntry is the string used in the license expression and the value is the URI for the corrosponding CustomLicense, CustomLicenseAddition, or SimpleLicensingText.
+
+
+## Metadata
+
+- name: customIdToUri
+- Nature: ObjectProperty
+- Range: /Core/DictionaryEntry