Add details about RTC / hwclock enabling

[shawaj]

Add details about RTC / hwclock enabling using config.txt for HAT version:

Name:   i2c-rtc
Info:   Adds support for a number of I2C Real Time Clock devices
Load:   dtoverlay=i2c-rtc,<param>=<val>
        mcp7940x                Select the MCP7940x device

Use following to load RTC driver for our RTC MCP7940N-I/SN: dtoverlay=i2c-rtc,mcp7940x

In future hardware versions enable auto-config using device tree if possible.

Also need documentation surrounding using hwclock functionality - see here for reference

[francesco-vannini]

This has been done in #55