[http] Document translation services need to be installed before usage (#16363)

* Update README.md

Signed-off-by: Li-Wen Yip <liwen.yip@gmail.com>

diff --git a/bundles/org.openhab.binding.http/README.md b/bundles/org.openhab.binding.http/README.md
index ef0995ebb6aec34511a04befec5717795661156f..720ab6dd84d76bf7bb41be12d9b5ba4e2608bc3c 100644 (file)
--- a/bundles/org.openhab.binding.http/README.md
+++ b/bundles/org.openhab.binding.http/README.md
@@ -66,6 +66,9 @@ When concatenating the `baseURL` and `stateExtension` or `commandExtension` the
 ### Value Transformations (`stateTransformation`, `commandTransformation`)
 
 Transformations can be used if the supplied value (or the required value) is different from what openHAB internal types require.
+
+The relevant transformation service needs to be installed via the Main UI or addons.cfg before use.
+
 Here are a few examples to unwrap an incoming value via `stateTransformation` from a complex response:
 
 | Received value                                                      | Tr. Service | Transformation                            |