README.dissectors: Add notes about predefined string structures not available to plugin authors

This issue was migrated from bug 13828 in our old bug tracker.

Original bug information:

Reporter: Dirk Weise
Status: RESOLVED FIXED
Product: Wireshark
Component: Documentation
OS: Windows 10
Platform: x86-64
Version: Git

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information