Update main README.md to show 0.7.0

[newtonick]

Since 0.7.0 is the latests release, the README.md needs some version updating.

[newtonick]

Tested:

$ gpg --fetch-keys https://keybase.io/seedsigner/pgp_keys.asc
gpg: requesting key from 'https://keybase.io/seedsigner/pgp_keys.asc'
gpg: key C7EF709007260119: "seedsigner <btc.hardware.solutions@gmail.com>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1
$ gpg --verify seedsigner.0.7.*.sha256.txt.sig
gpg: assuming signed data in 'seedsigner.0.7.0.sha256.txt'
gpg: Signature made Mon Jun 13 13:07:57 2022 EDT
gpg:                using RSA key 46739B74B56AD88F14B0882EC7EF709007260119
gpg: Good signature from "seedsigner <btc.hardware.solutions@gmail.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 4673 9B74 B56A D88F 14B0  882E C7EF 7090 0726 0119
$ shasum -a 256 --ignore-missing --check seedsigner.0.7.*.sha256.txt
seedsigner_os.0.7.0.pi0.img: OK
seedsigner_os.0.7.0.pi02w.img: OK
seedsigner_os.0.7.0.pi2.img: OK
seedsigner_os.0.7.0.pi4.img: OK

[jdlcdl]

ACK tested