add non linux warning to the readme.

Author: samuelcolvin

README.rst

@@ -12,6 +12,8 @@ pydf easier to use, in particular this means pydf works on heroku.
 
 Currently using **wkhtmltopdf 0.12.4 (with patched qt)**, requires **Python 3.6+**.
 
+**If you're not on Linux amd64:** pydf comes bundled with a wkhtmltopdf binary which will only work on Linux amd64 architectures. If you're on another OS or architecture your milage may vary, it is likely that you'll need to supply your own wkhtmltopdf binary and point pydf towards it by setting the `WKHTMLTOPDF_PATH` variable.
+
 Install
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