Add a sentence to public/README about where to start

Change-Id: I21e8724493eaa5e1ae766245bebdca3118e9187d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/93410
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

public/README

diff --git a/public/README b/public/README
index b07d0f3..5a8fd49 100644
--- a/public/README
+++ b/public/README
@@ -4,6 +4,8 @@
 included by an embedder of PDFium.  If there arises a need for functionality
 beyond what is present here, then a new API must be added here to provide it.
 
+Documentation for the API is within the headers. Start by reading fpdfview.h.
+
 These header files must be entirely contained in this directory; they must
 never include other header files from outside of it.