Emphasis needed to distinguish recovery image and recovery-installer image

[lschweiss]

In step 8, I accidentally used the ubi-initramfs-recovery_signed.itb image instead of ubi-initramfs-recovery-installer_signed.itb. This appeared to work until step 12, when the router rebooted and never came back.

This is not a unique problem. The YouTuber OneMarcFifty made the same mistake and detailed how to fix it here: https://youtube.com/watch?v=KgDbRJMG6uE

May I suggest a bold warning at step 8, noting the two different image types?

[dangowrt]

Please edit README.md (e.g. in Github's web editor) and make a pull request, I will review and accept your change.