Fix build steps in Readme.md

[astrogeco]

Is your feature request related to a problem? Please describe. The steps in readme are missing the copy sample_defs part

make SIMULATION=native prep
make
make install
cd build/exe/cpu1/
./core-cpu1

Describe the solution you'd like Delete, refer to another doc, or fix

[skliper]

The copy steps are in the setup section right above this.

[astrogeco]

Whoops, you're right. Maybe they should all be in a single section?

Or I was just too tired and it's obvious

[CDKnightNASA]

note that the ground system section could use a re-write; bad grammar...Should I take a crack at an update to README.md?

[CDKnightNASA]

And I assume the Apr. 1 date of this ticket is no indication as to the nature of the request. :D