update example in README

[aquynh]

for now README mentions this command:

$ stack run -- INPUT

but when replacing INPUT with a filename, it does not work:

$ stack run --  compiler/test-data/untyped/add0/add0.plc
clang: error: no such file or directory: './rts/rts.c'

[mbg]

Indeed. The compiler cannot easily know where the rts.c file is located relative to the location of the executable / working directory, so it defaults to ./rts/rts.c relative to the current working directory if no --rts is specified. If you are running the compiler from a directory where ./rts/rts.c doesn't exist, then you will see this error. Either navigate to a directory for which this assumption holds (i.e. compiler) or specify the path with either --rts ./compiler/rts/rts.c.

[aquynh]

yes, README should be updated to either explain the path issue or just remove that instruction.

i think it is good to provide some sample plc files (like in samples/ dir) and use them for demo in README.