[librsvg/rustification] tests/README.md: Mention the "ignore-" prefix for reftests

[Federico Mena Quintero <federico src gnome org>]

commit 3090becbf460d136e9db2f0bed2ccc9ab9942049
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Dec 13 21:16:55 2016 -0600

    tests/README.md: Mention the "ignore-" prefix for reftests

 tests/README.md |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/tests/README.md b/tests/README.md
index 25b38b0..0e984b3 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -63,6 +63,9 @@ directory; the `rsvg-test` program will recursively look inside
 `fixtures/reftests` for all SVG files, render them, and compare them to
 the `-ref.png` reference images.
 
+SVG test files in fixtures/reftests that match "ignore-*.svg" will be
+skipped from the tests.
+
 As of 2016/Nov/03 we have an informal organization of these files:
 
 * `fixtures/reftests/*.svg` - Tests for special situations