hdkeychain: Update README.md.
hdkeychain: Update README.md.

This updates the README.md in hdkeychain to account for changes made in
Decred, correct the NewMaster godoc example link, and format the
examples so the description is on the line after the link for
consistency with other packages in the code base.

decred/dcrd,jrick/dcrd,decred/dcrd,jrick/dcrd