Add docs for feature detection

docs/developing/feature-detection.md

@@ -0,0 +1,34 @@
+# Feature detection
+
+There are now multiple different versions of Sandstorm available in the
+wild. At present, most have fairly common code, and we expect most API
+features of Sandstorm to be widely supported. However, a developer may
+need to adjust app behavior based on support for different Sandstorm
+versions or runtimes.
+
+Sandstorm implementations are generally expected to run with an evergreen
+update model and automatic updates. This means developers should assume
+a given platform supports the latest version of the code for that
+distribution.
+
+The first community release of Sandstorm, build 309, introduces a new
+`SERVER_RUNTIME` environment variable, which returns `classic`. This
+indicates to the app that it is running on the modern community release
+of Sandstorm, and includes any features introduced after this fork. The
+absence of this environment variable should be treated as the feature set
+of the original Sandstorm.io platform, of which its final version was 308.
+
+When Tempest is available, it will return a value of `tempest`.
+
+## Implementing a fork with app platform features
+
+The Sandstorm project welcomes forks. If a forked codebase adds additional
+app platform API features, we recommend they [modify this value](https://github.com/sandstorm-io/sandstorm/pull/3766)
+which can only be changed in the source code. By encouraging apps to check
+the value of `SERVER_RUNTIME` prior to invoking custom API features, apps
+can be built which support multiple implementations of the platform, and
+are eligible to be included in and distributed by the Sandstorm app market.
+
+Do not use this feature as part of a proprietary app licensing scheme. The
+Sandstorm team may allow users to override this setting on specific apps if
+it is needed to bypass improper feature detection code.

mkdocs.yml

@@ -44,6 +44,7 @@ nav:
         - "Web publishing": "developing/web-publishing.md"
         - "Connecting grains to each other & Sandstorm via the powerbox": "developing/powerbox.md"
         - "Email from apps": "developing/email-from-apps.md"
+        - "Feature detection": "developing/feature-detection.md"
     - "App market publishing guide": "developing/publishing-apps.md"
 - Administering:
     - "Overview": "administering.md"