Add README.md for rendering PDFs in use_ahem/.

Change-Id: I00e7e264beea3f30b0abbdb44b333d37e76264c3
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/77750
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>

testing/resources/pixel/xfa_specific/use_ahem/README.md

diff --git a/testing/resources/pixel/xfa_specific/use_ahem/README.md b/testing/resources/pixel/xfa_specific/use_ahem/README.md
new file mode 100644
index 0000000..f3126e2
--- /dev/null
+++ b/testing/resources/pixel/xfa_specific/use_ahem/README.md
@@ -0,0 +1,2 @@
+The expected results in this directory should match those generated by running
+`pdfium_test` with the option `--font-dir=/path/to/pdfium/testing/resources/fonts`.