Update README.md

[spider1163]

cf. #214

[nemchik]

This is a good start, but the ATK folder might not always be in /opt/. Mine for example is in ~/.

[spider1163]

It is based on /opt folder you set in "Installation and Usage" section of README.
I assumed user who changed the git clone folder would understand the nuance.
I can however make it more general. What do you think of : [install-folder]/AtoMiC-ToolKit/backups ?

[nemchik]

The /opt/ folder was introduced as the default folder some time after I had initially installed ATK. Maybe something as simple as saying The default location for stored backups is /opt/AtoMiC-ToolKit/backups and as you said people who have installed it elsewhere probably know what to look for.

[nemchik]

Looks good to me.

[TommyE123]

@spider1163 Thanks ever so much for your contribution, Its very much appreciated.
Any further improvements are more than welcome :-)

@nemchik and I are both on Gitter if you wanted to discuss any ideas.

Cheers
Tom