readme: move up note about Page.pdf working on headless only (#577)
readme: move up note about Page.pdf working on headless only (#577)

Hard to find. From https://github.com/GoogleChrome/puppeteer/issues/576
ianchn/puppeteer,GoogleChrome/puppeteer,puppeteer/puppeteer,brutalcrozt/puppeteer,GoogleChrome/puppeteer,michaelfward/puppeteer,elisherer/puppeteer,elisherer/puppeteer,puppeteer/puppeteer,ianchn/puppeteer,brutalcrozt/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,a1ph/puppeteer,a1ph/puppeteer,michaelfward/puppeteer,GoogleChrome/puppeteer