Update README.asciidoc

I had a problem when using unoconv with Nginx and PHP-FPM.

I finally solved the issue with the help rendered here.

https://github.com/dagwieers/unoconv/issues/87#issuecomment-18800070

So I am adding the fix so that anyone after me can quickly realize the issue.
Merge pull request #140 from simkimsia/patch-1

Update README.asciidoc