add faq info for sub context

docs/tgbotapi/guides/faq.md

@@ -0,0 +1,32 @@
+# FAQ
+
+## How to filter updates in some part of `BehaviourBuilder`?
+
+You may create subcontext with
+`BehaviourBuilder.`[`createSubContext`](https://tgbotapi.inmo.dev/tgbotapi.behaviour_builder/dev.inmo.tgbotapi.extensions.behaviour_builder/create-sub-context.html)
+and pass there `updatesUpstreamFlow` parameter with any operations over parent behaviour builder:
+
+```kotlin
+buildBehaviourWithLongPolling {
+    createSubContext(
+        updatesUpstreamFlow = filter { /* some condition */ }
+    ) {
+        onCommand() //...
+    }
+}
+```
+
+### Additional info
+
+* [Flows docs](https://kotlinlang.org/docs/flow.html#intermediate-flow-operators)
+* [BehaviourBuilder](../logic/behaviour-builder.md)
+
+### Cases
+
+* Filtering of chats and users:
+    ```kotlin
+        updatesUpstreamFlow = filter { it.sourceChat() ?.id == requiredChatId || it.sourceUser() ?.id == requiredUserId }
+    ```
+    * **See**:
+        * [Update.sourceChat](https://tgbotapi.inmo.dev/tgbotapi.utils/dev.inmo.tgbotapi.extensions.utils.extensions/source-chat.html)
+        * [Update.sourceUser](https://tgbotapi.inmo.dev/tgbotapi.utils/dev.inmo.tgbotapi.extensions.utils.extensions/source-user.html)