add warning about headless batch upgrades

[psifertex]

If you open an old BNDB in Binary Ninja you will be asked whether to convert to the new format. Selecting "Yes" will automatically overwrite the old BNDB.

If you open a file via headless you are not given this choice and it will automatically be overwritten:

• [ ] headless should have a setting that lets you not automatically do this and if set it should trigger an exception
• [ ] this behavior should be better documented here