Add data guideline regarding constants (#8062)

See #7936 (comment)
and #7585 (comment)

docs/data-guidelines.md

@@ -13,6 +13,7 @@ This file contains recommendations to help you record data in a consistent and u
   - [`"partial_implementation"` requires a note](#partial_implementation-requires-a-note)
   - [Non-functional defined names imply `"partial_implementation"`](#non-functional-defined-names-imply-partial_implementation)
   - [Operating system limitations imply `"partial_implementation"`](#operating-system-limitations-imply-partial_implementation)
+  - [Constants](#constants)
   - [Release lines and backported features](#release-lines-and-backported-features)
   - [Safari for iOS versioning](#safari-for-ios-versioning)
   - [Addition of browsers](#addition-of-browsers)
@@ -177,6 +178,14 @@ However, this guideline does not apply to features where the browser's expected
 
 This guideline was proposed in [#6906](https://github.com/mdn/browser-compat-data/issues/6906).
 
+## Constants
+
+Don't include data for constants in BCD. The rationale for not including them is that they're not known to be a source of any compatibility issues.
+
+For example, although the UI Events specification defines a [`DOM_KEY_LOCATION_STANDARD`](https://w3c.github.io/uievents/#dom-keyboardevent-dom_key_location_standard) constant, we don't include data for it in BCD.
+
+This guideline was proposed in [#7936](https://github.com/mdn/browser-compat-data/issues/7585), based in part on previous discussion in [#7585](https://github.com/mdn/browser-compat-data/issues/7585).
+
 ## Release lines and backported features
 
 Use version numbers to reflect which _release line_ (major or minor but not patch-level releases) first supported a feature, rather than absolute version numbers.