diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -4,3 +4,23 @@
 Search service for the Software Heritage archive.
 
 Currently uses ElasticSearch, and provides only origin search (by URL and metadata)
+
+# Dependencies
+
+Python tests for this module include tests that cannot be run without a local
+ElasticSearch instance, so you need the ElasticSearch server executable on your
+machine (no need to have a running ElasticSearch server).
+
+## Debian-like host
+
+The following instructions is enough:
+```
+sudo apt install elasticsearch
+```
+
+
+## Non Debian-like host
+
+The tests expect:
+- `/usr/share/elasticsearch/jdk/bin/java` to exist.
+- `org.elasticsearch.bootstrap.Elasticsearch` to be in java's classpath.