Add browser removal/addition data guidelines

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 +174,30 @@ 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
+
+BCD's [owners](https://github.com/mdn/browser-compat-data/blob/master/GOVERNANCE.md) may choose to adopt a new browser or engine. 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)
+- a release process allowing BCD to publish stable release information in a `browsers/` file (containing release notes with version numbers and dates)
+- documentation (e.g., how to get and test a feature in that browser, links to resources that might help with it, etc.)
+
+This decision was proposed in [#7238](https://github.com/mdn/browser-compat-data/issues/7238) and adopted in [#7244](https://github.com/mdn/browser-compat-data/pull/7244).
+
+## Removal of browsers
+
+To maintain data quality, BCD's [owners](https://github.com/mdn/browser-compat-data/blob/master/GOVERNANCE.md) may choose to remove a browser or engine from the project. 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)
+
+Removing a browser from BCD does not constitute a ban; browsers may be readmitted under the [Addition of browsers](#addition-of-browsers) guideline.
+
+This decision was proposed in [#7238](https://github.com/mdn/browser-compat-data/issues/7238) and adopted in [#7244](https://github.com/mdn/browser-compat-data/pull/7244).
+
 ## 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: