Add checksums for the first two source releases for completeness.
These are copied from and verified against downloads.xiph.org.

We keep checksums of the release packages in the repo as a
redundant and trustworthy reference. I found these in an
old working repo; it looks like they weren't committed
after the 0.12 release last year.

Checked against the files on downloads.xiph.org.

We keep checksums of the release packages in the repo as a
redundant and trustworthy reference.

Verified against archive.mozilla.org and the original builds
on my machine.

Keep a copy of the release tarball SHA-256 checksums in the
repository for reference. Having a copy with the code in
distributed version control adds a verification path.

This should match the opusfile checksums in
https://archive.mozilla.org/pub/opus/SHA256SUMS.txt and
http://downloads.xiph.org/releases/opus/SHA256SUMS.txt