Add HINTS.md for hello-world/difference-of-squares/rna-transcription to address issue #234 (#243)

Author: robkeim

exercises/difference-of-squares/HINTS.md

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+## Hints
+For this exercise the following F# feature comes in handy:
+- [Seq.sumBy](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/seq.sumby%5B't,%5Eu%5D-function-%5Bfsharp%5D) is a condensed format to apply a function to a sequence and then sum the results

exercises/grains/HINTS.md

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 ## Hints
 For this exercise the following F# feature comes in handy:
-- [BigInt](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/numerics.biginteger-structure-%5Bfsharp%5D)
+- [BigInt](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/numerics.biginteger-structure-%5Bfsharp%5D)
+- [Seq.sumBy](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/seq.sumby%5B't,%5Eu%5D-function-%5Bfsharp%5D) is a condensed format to apply a function to a sequence and then sum the results

exercises/hello-world/HINTS.md

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+## Hints
+For this exercise the following F# feature comes in handy:
+- [Match Expressions](https://fsharpforfunandprofit.com/posts/match-expression/) Also known as pattern matching which is like a switch statement on steriods. It's a very powerful ability in the F# language and will come in handy in many of the exercises. While this example can be trivially solved with an if/else, pattern matching is more idiomatic.
+- [sprintf](https://fsharpforfunandprofit.com/posts/printf/) While F# supports string concatenation using sprintf leads to easier to read solutions and is also idiomatic
+- [defaultArg](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/operators.defaultarg%5B't%5D-function-%5Bfsharp%5D) Default arg is useful when extracting a value from an option type and providing a value when None is encountered.

exercises/rna-transcription/HINTS.md

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+## Hints
+For this exercise the following F# feature comes in handy:
+- [Match Expressions](https://fsharpforfunandprofit.com/posts/match-expression/) While this can be solved using a dictionary, using a match expression is more idiomatic.