Add file that documents the thought behind problem order

problems.md

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+# Rust Track Problem Order
+
+The actual source of truth of problem order is [config.json](config.json), but this file documents our reasoning behind the problem order in that file.
+
+## Background
+
+- https://github.com/exercism/xrust/issues/126
+- https://github.com/exercism/xrust/issues/127
+- http://designisrefactoring.com/2016/07/09/exercism-shouldnt-make-you-cry/
+
+# The Problems, By Section
+
+## Introduction
+
+The first section contains the sort of stuff you expect when learning any programming languages: conditionals, booleans, looping and some higher-order functions.
+
+problem | topics
+----- | -----
+hello-world |  Some/None. println!
+gigasecond |  crates, math
+leap |  math, booleans, conditionals
+raindrops |  case (or `format`). Mutable string
+bob |  chars, string functions
+beer-song |  case, string concatenation, vector (optional), loop
+difference-of-squares |  fold & map
+
+## Getting Rusty
+
+Problems begin to use more Rust-specific features. Try to only introduce one new language feature at a time. And if several problems rely on a feature, try to group them so as to reinforce its usage.
+
+problem | topics
+----- | -----
+hamming |  Result
+scrabble-score |  chaining higher-order functions, HashMap (optional)
+pangram | filter, ascii (optional)
+nucleotide-count |  filter, entry api, mutablity, match
+word-count |  hashmap, str vs string, chars, entry api
+etl |  btree
+sieve |  vector, map, while let (optional)
+rna-transcription |  match, struct, str vs string
+roman-numerals |  mutable, results, loops, struct, traits
+hexadecimal |  Option, zip/fold/chars, map
+grade-school |  struct, entry api, Vec, Option
+tournament |  enum, sorting, hashmap, structs
+robot-simulator | Immutability, enum
+queen-attack |  struct, trait (optional), Result
+sublist |  enum, generic over type
+allergies |  struct, enum, bitwise (probably), vectors, filter
+variable-length-quantity | Encodings, slices, bitwise, Result
+phone-number |  option, format, unwrap_or, iters, match
+wordy | Result, string parsing, operators (optional)
+custom-set |  generic over type, vector, equality, struct
+
+## Rust Gets Strange
+
+Exercises that pay the cost of Rust's [strangeness budget](http://words.steveklabnik.com/the-language-strangeness-budget). Features that are very specific to Rust.
+
+problem | topics
+----- | -----
+anagram |  lifetimes, str vs string, loops, iter, vector
+nucleotide-codons |  struct, hash map, lifetimes, Result
+robot-name |  struct, slices, randomness, lifetimes, self mut
+
+## Putting it all Together
+
+These problems don’t necessarily require additional Rust knowledge, but they do require complex solutions.
+
+problem | topics
+----- | -----
+minesweeper |  Board state
+dominoes |  Graph theory, searching
+parallel-letter-frequency | multi-threading
+rectangles |  Enum, structs, traits, Lifetimes
+forth |  Parser reimplementation
+circular-buffer |  Buffer reimplementation, Generics