Document version, build flags and patches for bundled pdfium.dll

dependencies/pdfium/win/VERSION

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+YACReader for Windows uses a shared library version of pdfium.
+
+pdfium branch used for building: chromium/3729
+
+Build parameters used (gn args):
+
+pdf_enable_v8 = false
+pdf_enable_xfa = false
+is_debug = false
+symbol_level = 0
+use_jumbo_build = true
+
+For 32 bit builds:
+target_cpu="x86"
+
+For 64 bit builds:
+target_cpu="x64"
+
+Instructions on building pdfium can be found at https://pdfium.googlesource.com/pdfium
+
+To build a shared library, the source code has to be patched. A patch file with the
+necessary modifications can be found in this directory (pdfium_shared_library.patch).
+
+It is recommended to always use the branch the current stable version of Chromium uses.
+To get the pdfium branch corresponding to Chromium stable, look at 
+http://omahaproxy.appspot.com and search for the true_branch variable associated with
+the current stable dev channel of Chromium.