Should make signature verification instructions more beginner-friendly

[ddawson]

In the section, "How to verify GPG signatures", there is the following:

In order to be able to verify GPG signatures, you need to import the public key of the signer. Electrum binaries are signed with ThomasV's public key. On Linux, you can import that key using the following command: gpg --import ThomasV.asc.

This is a little unclear. Someone who doesn't know how to use GnuPG might not realize that "ThomasV.asc" is just a placeholder for however they input the key, as this Reddit post demonstrates. I suggest rewriting it to make it clearer. Also, watch the order of instructions, as it appears to ask them to verify the signature before importing the key.