Update README with instructions

README.md

@@ -1,3 +1,39 @@
 # tc-calc
 
 Our built-in calculator. This is the REPL loop used by Terminal Click.
+
+## Build Instructions
+
+`cc main.c -o calc -lm`
+
+Where `cc` is our C compiler e.g. `gcc`, `clang`, `zig cc`.
+
+## How to Use
+
+Run the executable `calc` to be greeted with the following prompt:
+
+`calc>`
+
+Now we type any valid arithmetic expression as we would in a Python REPL:
+
+```
+calc> (2+2)*(2+2)
+  16
+calc>
+```
+
+Notice how we print the output with a bit of spacing followed by a new prompt.
+
+**Switching Formats**
+
+We can switch between decimal, hexadecimal and binary formats. Use the `:dec`, `:hex`, or `:bin` tags:
+
+```
+calc> :hex
+  format: hex
+calc> 0xA - 0x9
+  0x1
+calc>
+```
+
+The default tag is `:dec`.