deps: add example of comparing OpenSSL changes

When upgrading OpenSSL, Step 6 in upgrading guide explains the steps
that need to be taken if asm files need updating. This might not
always be the case and something that needs to be checked from
release to release.

This commit adds an example of using github to manually compare two tags
to see if any changes were made to asm files.

PR-URL: https://github.com/nodejs/node/pull/13234


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>