Add an article about free transactions

[dagurval]

Motivated by https://github.com/bitcoinxt/bitcoinxt/pull/461

Pushed to website for easier review: https://bitcoinxt.software/doc/freetransactions.html

[dagurval]

• Added a date on article
• Made calculator increase total transaction size when input is added
• Added a comment about still accepting free transactions to mempool when blocks are full
• Added a reference to Satoshis original commit for priority threshold

[dgenr8]

Post-hoc ACK. This is fantastic.

[dagurval]

Thanks @dgenr8 !