Add browser removal/addition data guidelines

This change adds data guidelines on removal of existing browsers from
BCD, and addition of new browsers to BCD.

Related: mdn#7238

docs/data-guidelines.md

@@ -13,6 +13,8 @@ This file contains recommendations to help you record data in a consistent and u
   - [Non-functional defined names imply `partial_implementation`](#non-functional-defined-names-imply-partial_implementation)
   - [Release lines and backported features](#release-lines-and-backported-features)
   - [Safari for iOS versioning](#safari-for-ios-versioning)
+  - [Addition of browsers](#addition-of-browsers)
+  - [Removal of browsers](#removal-of-browsers)
   - [Removal of irrelevant features](#removal-of-irrelevant-features)
   - [Removal of irrelevant flag data](#removal-of-irrelevant-flag-data)
 
@@ -172,6 +174,24 @@ For Safari for iOS, use the iOS version number, not the Safari version number or
 
 This versioning scheme came at [Apple's request, in #2006](https://github.com/mdn/browser-compat-data/issues/2006#issuecomment-457277312).
 
+## Addition of browsers
+
+To add a new browser to BCD, we need evidence of (in decreasing order of importance):
+
+- a compelling downstream-consumer story (e.g., MDN or caniuse express an interest, or someone is planning to do something with the data that might plausibly grow BCD's reach)
+- reviewers (e.g., two or more people with interest and ability to test data relating to new and existing releases, or at least one reviewer acting on behalf of the vendor in BCD)
+- published release information (e.g., release notes with version numbers and dates, etc.)
+- documentation (e.g., how to get and test a feature in that browser, links to resources that might help with it, etc.)
+
+## Removal of browsers
+
+To remove a browser from BCD, we need habitual (six months or more) evidence of (in decreasing order of importance):
+
+- negative/neutral downstream-consumer interest in the browser's data (e.g., MDN and caniuse don't object to removal)
+- poor data coverage with negative trends (e.g., our data for the browser covers only a few features, with limited/flat growth in more data being added for it, or few features with real version numbers rather than just `null` or `true`, etc.)
+- infrequent community or vendor involvement in issues or PRs relating to the browser
+- infrequent new PRs relating to the browser (e.g., weeks or months go by without PRs touching the browser's data)
+
 ## Removal of irrelevant features
 
 Features can be removed from BCD if it is considered irrelevant. A feature can be considered irrelevant if any of these conditions are met: