Add generated documentation to .gitignore

Change-Id: I5175ba1b316e381949c820211769fbd636fe645e
Reviewed-on: https://boringssl-review.googlesource.com/3024
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/.gitignore b/.gitignore
index 58995c5..b54125f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 ssl/test/runner/runner
 *.swp
 *.swo
+doc/*.html