Add README

README.md

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+# [The Universal Calculator](https://git.ewanick.com/bill/universal-calculator)
+
+Using the GHC REPL and Haskell to teach programming concepts through the guise of mathematics.  
+A handy assistant to help you understand and solve maths problems.
+
+- GHC = [Glasgow Haskell Compiler](https://www.haskell.org/ghc/)
+- REPL = [Read Evaluate Print Loop](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop)
+- Haskell = [Lazy, Pure, Functional Programming Language](https://www.haskell.org/)
+
+## How to use
+
+```Haskell
+λ> 5 * 5
+25
+
+λ> 1 + 5
+6
+
+λ> 10 / 2
+5.0
+
+λ> 10 `div` 2
+5
+
+λ> 10 `div` 3
+3
+
+λ> 10 / 3
+3.3333333333333335
+
+λ> :t sum
+sum :: (Foldable t, Num a) => t a -> a
+
+λ> :t sum [1..100]
+sum [1..100] :: (Num a, Enum a) => a
+
+λ> sum [1..100]
+5050
+
+λ> f x = 5*x^3 + 4*x + 2
+
+λ> f 1
+11
+
+λ> f 10
+5042
+
+λ> f 100
+5000402
+
+λ> map f [1..10]
+[11,50,149,338,647,1106,1745,2594,3683,5042]
+
+λ> putStrLn "Hello, world!"
+Hello, world!
+```
+
+## How to install
+
+- Use the DeterminateSystems installer to install Nix with Flakes.
+  - <https://github.com/DeterminateSystems/nix-installer>
+- `cd` into the top level of this repo.
+- `nix develop` and let Nix load the flake and set everything up.
+- `ghci` and get computing!
+  - To disable performance timings after every input, type `λ> :unset +s` in the REPL
+  - See other initial commands being run in the `.ghci` file.