Update README.md

README.md

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 # CLIcalc.nim
-A command-line calculator in nim. (reimplementation of CLIcalc.jl)
+A command-line calculator in nim. [(reimplementation of CLIcalc.jl)](https://github.com/A-childs-encyclopedia/CLIcalc.jl/)
+
+# Usage instructions
+1. Enter a function <br/>
+   1a. mul = Multiplies x and y <br/>
+   1b. sub = Subtracts y from x <br/>
+   1c. add = Adds x and y <br/>
+   1d. div = Divides x by y (result will be a float) <br/>
+   1e. exit = Exits program <br/>
+2. When prompted, supply an **integer** value for x and y
+3. When you have finished using the program, use the exit function
+
+# Installation instructions on Linux
+1. Download latest release
+2. Run ./CLIcalc in download directory
+
+# Installation instructions on Windows/Mac
+1. [Install nim](https://nim-lang.org/install.html)
+2. Run `git clone https://github.com/A-childs-encyclopedia/CLIcalc.nim`
+3. Run `cd CLIcalc.nim`
+4. Run `nimble build -d:release`
+5. <br/>
+   5a. On Windows, run `.\CLIcalc` <br/>
+   5b. On Mac, run `./CLIcalc`
+
+## PRs are welcome :)