pyln-spec: add .gitignore file for build detritus.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-spec/.gitignore

@@ -0,0 +1,12 @@
+bolt1/build/
+bolt1/dist/
+bolt1/pyln_bolt1.egg-info/
+bolt2/build/
+bolt2/dist/
+bolt2/pyln_bolt2.egg-info/
+bolt4/build/
+bolt4/dist/
+bolt4/pyln_bolt4.egg-info/
+bolt7/build/
+bolt7/dist/
+bolt7/pyln_bolt7.egg-info/