RUMM-1323 Undefined swift symbols added to troubleshooting.md

TROUBLESHOOTING.md

@@ -1,5 +1,26 @@
 # TROUBLESHOOTING
 
+### `Undefined symbols: Swift`
+
+Original issue: https://github.com/DataDog/dd-sdk-reactnative/issues/41
+
+If you have the following error message:
+```
+Undefined symbols for architecture x86_64:
+  "static Foundation.JSONEncoder.OutputFormatting.withoutEscapingSlashes.getter : Foundation.JSONEncoder.OutputFormatting", referenced from:
+      static (extension in Datadog):Foundation.JSONEncoder.default() -> Foundation.JSONEncoder in libDatadogSDK.a(JSONEncoder.o)
+...
+```
+
+open Xcode and go to `Build Settings` of your project (_not your app target_) then make sure `Library Search Paths` is the following:
+```
+LIBRARY_SEARCH_PATHS = (
+  "\"$(TOOLCHAIN_DIR)/usr/lib/swift/$(PLATFORM_NAME)\"",
+  "\"/usr/lib/swift\"",
+  "\"$(inherited)\"",
+);
+```
+
 ### `Undefined symbol _RCTModule`
 
 I had this issue when I run the project from Xcode. It might be related to this change in [`react-native v0.63 changelog`](https://github.com/facebook/react-native/commit/6e08f84719c47985e80123c72686d7a1c89b72ed)