improving README for the missing libray issue at installation

[jlmenut]

Hello,

I would suggest to precise in the README that when installing nimlsp with the option -d:explicitSourcePaththe path must be absolute, not relative and that the nim version to clone is the one corresponding to the actual version installed . It may be obvious for a lot of persons but not everyone and it's not a big change

[PMunch]

Good point, and an easy enough thing to fix.