Update README - gplay submodule

[guppy42]

The read me step "checkout the repository" is a tad too vague

what is needed is;

git clone https://github.com/MCMrARM/mcpelauncher-linux.git
cd mcpelauncher-linux/
git submodule init
git submodule update

not including line 3+4 (which are less than obvious if you've not worked with submodules before) will cause a cmake error complaining about gplay_api no containing a CMakeList.txt file

[MCMrARM]

git clone --recursive https://github.com/MCMrARM/mcpelauncher-linux.git should do the job as well

[guppy42]

fair point sadly it seems you cannot mix code tags and markup - I would have like for the --recursive part to be in bold or something to make sure people paid attention to it.