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diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index 342bc1d340..d07790f184 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -24,6 +24,8 @@ The code of Synapse is written in Python 3. To do pretty much anything, you'll n
 
 Synapse can connect to PostgreSQL via the [psycopg2](https://pypi.org/project/psycopg2/) Python library. Building this library from source requires access to PostgreSQL's C header files. On Debian or Ubuntu Linux, these can be installed with `sudo apt install libpq-dev`.
 
+Synapse has an optional, improved user search with better Unicode support. For that you need the development package of `libicu`. On Debian or Ubuntu Linux, this can be installed with `sudo apt install libicu-dev`.
+
 The source code of Synapse is hosted on GitHub. You will also need [a recent version of git](https://github.com/git-guides/install-git).
 
 For some tests, you will need [a recent version of Docker](https://docs.docker.com/get-docker/).