download: add links for checksum and signature files.

Author: ericonr

download/index.markdown

@@ -21,3 +21,6 @@ downloaded image or tarball before using it, to ensure it hasn’t been tampered
 with. [Instructions on how to do
 that](https://docs.voidlinux.org/installation/index.html#verifying-images)
 are provided in the [Void Handbook](https://docs.voidlinux.org/).
+
+It will be necessary to download the [checksum file]({{ site.download_mirror }}/sha256sums.txt)
+and its [signature]({{ site.download_mirror }}/sha256sums.txt) for this step.