.gitignore was ignoring a documentation directory so added am explicit pattern retain it

Change-Id: I5db8220a95bc0a0b2b360db5588d70ef763e4599
diff --git a/.gitignore b/.gitignore
index e65eff4..9e8406b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,8 @@
 voltha-protos
 voltha-system-tests
 voltctl
+!howto/release/voltctl
+
 cord-tester
 device-management-interface