Add develop docs documentation

Author: cmanallen

develop-docs/sdk/expected-features/index.mdx

@@ -91,6 +91,27 @@ Local variable names and values for each stack frame, where possible. Restrictio
 
 This functionality should be gated behind the `includeLocalVariables` option, which is `true` by default.
 
+## Feature Flags
+
+An SDK may expose a Scope property for tracking feature flag evaluations. When the scope forks its important to clone the feature flags property. Leaking flag evaluations between threads could lead to inaccurate feature flag evaluation logs.
+
+The Scope's flag property should have a capped capacity and should prefer recently-evaluated flags over less-recently-evaluated flags. The recommended data structure is a LRU-cache but it is not required so long as the data structure behaves similarly. Serious deviations from the behavior of an LRU-cache should be documented for your language.
+
+The Scope's flag property should expose two methods: `get/0` and `set/2`. `set/2` takes two arguments. The first argument is the name of the flag. It is of type string. The second argument is the evaluation result. It is of type boolean. `set/2` should remove all entries from the LRU-cache which match the provided flag's name and append the new evaluation result to the end of the queue. `get/0` accepts zero arguments. The `get/0` method must return a list of serialized flag evaluation results in order of evaluation. Oldest values first, newest values last. See the <Link to="/sdk/data-model/event-payload/contexts/#feature-flag-context">Feature Flag Context</Link> protocol documentation for details.
+
+### Integrations
+
+Integrations automate the work of tracking feature flag evaluations and serializing them on error context. An integration should hook into third-party SDK and record feature flag evaluations using the current scope. For example, in Python an integration would call `sentry_sdk.get_current_scope().flags.set(...)` on each flag evaluation.
+
+An integration is also responsible for registering an "on error" hook with the Sentry SDK. When an error occurs the integration should request the current scope and serialize the flags property. For example, in Python an integration might define this callback function:
+
+```python
+def flag_error_processor(event, exc_info):
+    scope = sentry_sdk.get_current_scope()
+    event["contexts"]["flags"] = {"values": scope.flags.get()}
+    return event
+```
+
 ## Desymbolication
 
 Turn compiled or obfuscated code/method names in stack traces back into the original. Desymbolication always requires Sentry backend support. Not necessary for many languages.