Documentation staticRead

[juancarlospaco]

• Documentation of staticRead and slurp maximum file size limits.
• Small change, easy merge, only doc.

Someone was trying to staticRead a 9 gigabytes file on a 8 gigabytes notebook,
just adds a line on the documentation explaining this for new people learning the lang.

Tested on multiple PC with different RAM and devel/stable with:

static:  # nim c -d:gigas=8 file.nim
  const gigas {.intdefine.} = 8
  doAssert gorgeEx("fallocate -l " & $gigas & "G " & $gigas).exitCode == 0
  defer: doAssert gorgeEx("rm " & $gigas).exitCode == 0
  doAssert staticRead($gigas).len > 0