Change PRIVATE_KEY to MNEMONIC in env file

[colinsteidtmann]

The bash environment variable names (in the readme) are inconsistent with the env file. Rename PRIVATE_KEY as MNEMONIC in the env and example env files, or, add them in addition to PRIVATE_KEY.

[PatrickAlphaC]

Thanks for making this! Addressing.

[pappas999]

This is addressed in #36, it's been decided that having/using PRIVATE_KEY is preferred over MNEMONIC, so everything has been updated to reflect this