chore: add new issue template for CDE implementation requests

.github/ISSUE_TEMPLATE/new_cde.md

@@ -0,0 +1,21 @@
+---
+name: Addition of a Common Data Element (CDE)
+about: A request to add an approved Common Data Element (CDE) to the API specification.
+title: "CDE Implementation Request: Library Source Material"
+labels: ""
+assignees: ""
+---
+
+_(Please replace the example content below with your new requested CDE.)_
+
+-   **An informal CDE name**: Library Source Material
+-   **CDE number in caDSR, including version**: 15235975
+-   **Deeplink to CDE in caDSR**: [click here](https://cadsr.cancer.gov/onedata/dmdirect/NIH/NCI/CO/CDEDD?filter=CDEDD.ITEM_ID=15235975%20and%20ver_nr=1).
+-   **Link to the GitHub discussion where this CDE was ratified**: [click here](https://github.com/CBIIT/ccdi-federation-api/discussions/119).
+
+### Notes
+
+Anything unique about this CDE that the implementer should be aware of. For
+example, are we using only a subset of the caDSR values? Do the values have some
+unit? Are there a large number of possible values that we can't concisely
+enumerate?