Update README.md to mention V8 and XFA requires C++14.

Well, not just yet, but in the near future.

Bug: pdfium:1407
Change-Id: I55f829bc85d74d4ac233daeab48ffa303aeba335
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/61930
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/README.md b/README.md
index de86081..75abf8b 100644
--- a/README.md
+++ b/README.md
@@ -110,8 +110,8 @@
 For sample applications like `pdfium_test` to build, one must set
 `pdf_is_standalone = true`.
 
-To use the Skia backend, one must set `use_cxx11 = false` which will build the
-entire project with C++14.
+To enable V8, XFA, or the Skia backend, one must set `use_cxx11 = false` which
+will build the entire project with C++14.
 
 When complete the arguments will be stored in `<directory>/args.gn`, and
 GN will automatically use the new arguments to generate build files.