Improve README to illustrate clearer usage of the -a/-e options

[david-homley]

The README.md is not clear on how to use the -a/-e options, e.g. there are no clear examples documenting their expected usage.

The 'help' message could also be a bit more descriptive and clear in stating the expected format of the supplied arguments as well.

[al-jeyapal]

Specifically, you need to supply values in the format HELLO=/path/to/hello. Our help text should just explain that better. Especially in relation to -a since the key (left hand side) will be used as an environment variable name, and needs to conform to certain conventions (cannot contain hyphens, etc)

[thomas-anderson-bsl]

Resolved by #45