Extra explanation for getchaintips
Add 0.10 release notes on improvement to signing security.

I dropped mention of libgmp that I had in my first draft because
it looks like we'll be able to get that out prior to release.
Merge pull request #5406

5fdbe67 Add 0.10 release notes on improvement to signing security. (Gregory Maxwell)
Merge pull request #5400

32b93a1 Extra explanation for getchaintips (Pieter Wuille)
Merge remote-tracking branch 'upstream/master'