"Known limitations" section of README is incomplete

[RyanGlScott]

While looking at the code in Language.Haskell.TH.Desugar.Core recently, it struck me that there are quite a few language constructs that th-desugar does not support, but several of these are not mentioned in the th-desugar README's "Known limitations" section. Notable omissions include:

• Lack of support for RecursiveDo:

  th-desugar/Language/Haskell/TH/Desugar/Core.hs

  Line 243 in ed1d98c

  dsExp (MDoE {}) = fail "th-desugar currently does not support RecursiveDo"

• Lack of support for ApplicativeDo (see dsDoStmts doesn't support ApplicativeDo #138)

• Lack of support for view patterns (see Pattern guards are not a sufficient replacement for view patterns #174):

  th-desugar/Language/Haskell/TH/Desugar/Core.hs

  Lines 652 to 653 in ed1d98c

  	dsPat (ViewP _ _) =
  	fail "View patterns are not supported in th-desugar. Use pattern guards instead."

• No support for unresolved infix operators:

  th-desugar/Language/Haskell/TH/Desugar/Core.hs

  Lines 79 to 80 in ed1d98c

  	dsExp (UInfixE _ _ _) =
  	fail "Cannot desugar unresolved infix operators."