Added note about opcache

Added information about problems caused by opcache. As I'm not native English speaker, I'm not really sure that I have phrased it correctly. However I'm sure that there is need for this or similar message in README.
Merge pull request #30 from Endijs/patch-1

Added note about opcache
[#30] Rephrase note about opcode caches
Merge branch 'hotfix/30'

Close #30