docs: clarify the issues of monadic DSL

[doofin]

Hi, I see the readme says that the below code doesn't work

for {
  textA <- read(fileA)
  // Not a possible syntax
  if (fileA.contains("some string")) {
    textB <- read(fileB)
    _ <- write(fileC, textA + textB)
  }
} yield ()

However it seems that the code below works:

val o1 = Option(1)
val o2 = Option(1)

// condition in monadic DSL
for {
  a <- o1
  _ <- if true then o1 else o2
} yield ()

// while loop in monadic DSL
for {
  a <- o1
  _ <- Option(while (true) {})
} yield ()

so it seems that control structure is actually supported in the monadic DSL?