Add guidance on spacing with a unit of measurement

[julian-cable]

Guidance to add:
"Use a non-breaking space between a numeral and a unit of measurement."

Original example:
"The crashkernel=auto setting requires at least 1GB of memory."

Improvement example:
"The crashkernel=auto setting requires at least 1{nbsp}GB of memory."

[julian-cable]

Added to guidance about non-breaking spaces.