Added requirements.txt for the package

Signed-off-by: Grzegorz Latosinski <glatosinski@antmicro.com>

requirements.txt

diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..ab90481
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+lxml