Merge pull request #1417 from Unleash/docs/add-offline-info-to-sdks

docs: describe how sdks work in offline modes

website/docs/sdks/index.md

@@ -117,9 +117,26 @@ Here's some of the fantastic work our community has done to make Unleash work in
 - [uekoetter.dev/unleash-client-dart](https://pub.dev/packages/unleash) (Dart)
 - _...your implementation for your favorite language._
 
-## Implement your own SDK {#implement-your-own-sdk}
+### Implement your own SDK {#implement-your-own-sdk}
 
 If you can't find an SDK that fits your need, you can also develop your own SDK. To make implementation easier, check out these resources:
 
 - [Unleash Client Specifications](https://github.com/Unleash/client-specification) - Used by all official SDKs to make sure they behave correctly across different language implementations. This lets us verify that a gradual rollout to 10% of the users would affect the same users regardless of which SDK you're using.
 - [Client SDK overview](../client-specification) - A brief, overall guide of the _Unleash Architecture_ and important aspects of the SDK role in it all.
+
+## Working offline
+
+Once they have been initialised, all Unleash clients will continue to work perfectly well without an internet connection or in the event that the Unleash Server has an outage.
+
+Because the SDKs and the Unleash Proxy cache their feature toggle states locally and only communicate with the Unleash server (in the case of the server-side SDKs and the Proxy) or the Proxy (in the case of front-end SDKs) at predetermined intervals, a broken connection only means that they won't get any new updates.
+
+Unless the SDK supports [bootstrapping](#bootstrapping) it *will* need to connect to Unleash at startup to get its initial feature toggle data set. If the SDK doesn't have a feature toggle data set available, all toggles will fall back to evaluating as disabled or as the specified default value (in SDKs that support that).
+
+### Bootstrapping
+
+By default, all SDKs reach out to the Unleash Server at startup to fetch their toggle configuration. Additionally some of the server-side SDKs and the Proxy (see the above [compatibility table](#server-side-sdk-compatibility-table)) also support *bootstrapping*, which allows them to get their toggle configuration from a file, the environment, or other local resources. These SDKs can work without any network connection whatsoever.
+
+Bootstrapping is also supported by the following front-end client SDKs:
+- [the JavaScript proxy client](/sdks/proxy-javascript)
+- [the React Proxy client](/sdks/proxy-react)
+- [the Android proxy client](/sdks/proxy-react)