docs: add details of OpenFeature to LaunchDarkly context mapping (#531)

Signed-off-by: Zellyn Hunter <[email protected]>

Author: zellyn

providers/launchdarkly/README.md

@@ -17,5 +17,66 @@ go get github.com/open-feature/go-sdk-contrib/providers/launchdarkly/pkg
 ## Usage
 See [example_test.go](./example_test.go)
 
+## Representing LaunchDarkly (multi) contexts
+
+The LaunchDarkly provider expects contexts to be either single- or
+multi-context, matching [LaunchDarkly's concept of Contexts](https://docs.launchdarkly.com/guides/flags/intro-contexts).
+The representation of LaunchDarkly context(s) within the OpenFeature
+context needs to be well-formed.
+
+### Single context
+
+```JSON
+{
+  // The "kind" of the context. Required.
+  // Cannot be missing, empty, "multi", or "kind".
+  // Must match `[a-zA-Z0-9._-]*`
+  // (The default LaunchDarkly kind is "user".)
+  kind: string,
+
+  // The targeting key. One of the following is required to be
+  // present and non-empty. If both are present, `targetingKey`
+  // takes precedence.
+  key: string,
+  targetingKey: string,
+
+  // Private attribute annotations. Optional.
+  // See https://docs.launchdarkly.com/sdk/features/private-attributes
+  // for the formatting specifications.
+  privateAttributes: [string],
+
+  // Anonymous annotation. Optional.
+  // See https://docs.launchdarkly.com/sdk/features/anonymous
+  anonymous: bool,
+
+  // Name. Optional.
+  // If present, used by LaunchDarkly as the display name of the context.
+  name: string|null,
+
+  // Further attributes, in the normal OpenFeature format.
+  // Attribute names can be any non-empty string except "_meta".
+  //
+  // Repeated `string: any`
+}
+```
+
+### Multi context
+
+```JSON
+{
+  // The "kind" of the context. Required.
+  // Must be "multi".
+  kind: "multi",
+
+  // Sub-contexts. Each further key is taken to be a "kind" (and
+  // thus must match `[a-zA-Z0-9._-]`).
+  // The value is should be an object, and is processed using the
+  // rules described in [Single context](#single-context) above,
+  // except that the "kind" attribute is ignored if present.
+  //
+  // Repeated `string: object`
+}
+```
+
 ### References
 * https://docs.openfeature.dev/blog/creating-a-provider-for-the-go-sdk/