Update Readme.md

Author: Ilya Skvortsov

tests/Readme.md

@@ -13,6 +13,9 @@ Check physical correctness of restored instances:
 Check archive compression:
  export ARCHIVE_COMPRESSION=ON
 
+Specify path to pg_probackup binary file. By default tests use <Path to Git repository>/pg_probackup/
+ export PGPROBACKUPBIN=<path to pg_probackup>
+
 Usage:
  pip install testgres
  pip install psycopg2