[docs] how-to-submit-a-transaction.md suggestion / change request

[b1bart]

File: [docs/how-to-guides/01_how-to-submit-a-transaction.md](https://github.com/EOSIO/eosjs/blob/develop/docs/how-to-guides/01_how-to-submit-a-transaction.md) This example and the README use a `blocksBefore` of `3`. It seems like `useLastIrreversible` is a safer default for all how-to guides with the `blocksBefore` being something discussed as an additional option. This has the added benefit of generating the `refBlockNum` and `refBlockPrefix` based on the results of `get_info` rather than issuing an additional `get_block`

[gleehokie]

Same change needs to be made to https://github.com/EOSIO/eosjs/blob/master/README.md. There may be other docs too that should be changed.

Note, this can prevent some of the micro forking + invalid TAPOS related issues that have been reported.