[ANNOUNCE] NSPR 4.28 Release

[Kai Engert]

The NSPR 4.28 release is now available. The hg tag is
NSPR_4_28_RTM. The source tar file can be downloaded from
https://ftp.mozilla.org/pub/mozilla.org/nspr/releases/v4.28/src/

NSPR 4.28 contains the following changes:
- Fix a compiler warning
- Add rule for cross-compiling with cygwin

For details, please refer to the list of related bugs:
https://bugzilla.mozilla.org/buglist.cgi?resolution=FIXED&query_format=advanced&product=NSPR&target_milestone=4.28

Thanks to all contributors to this release.
Kai