Added some dev instructions to README

README.md

@@ -1,3 +1,60 @@
 calculation + conversion
 
 Uses Decimal Floating Point numbers instead of Binary Coded Decimals for better accuracy.
+
+# Dev Instructions
+
+## Get started
+Install [Rust](https://www.rust-lang.org). This project was built in Rust 1.40.
+
+## Commands
+Run cpc with a CLI argument as input:
+```
+cargo run -- '100ms to s'
+```
+
+Run tests:
+```
+cargo test
+```
+
+## Adding a unit
+### 1. Add the unit
+In `src/units.rs`, units are specified like this:
+```rs
+pub enum UnitType {
+  Time,
+  // etc
+}
+
+// ...
+
+create_units!(
+  Nanosecond:         (Time, d128!(1)),
+  Microsecond:        (Time, d128!(1000)),
+  // etc
+)
+```
+
+The number associated with a unit is it's "weight". For example, if a second's weight is `1`, then a minute's weight is `1000`.
+
+I have found [translatorscafe.com](https://www.translatorscafe.com/unit-converter) to be a good website for accurate and comprehensive unit convertion. Wikipedia is worth looking at as well.
+
+### 2. Add a test for the unit
+Make sure to also add a test for each unit. The tests look like this:
+```rs
+assert_eq!(convert_test(1000.0, Meter, Kilometer), 1.0);
+```
+Basically, 1000 Meter == 1 Kilometer.
+
+### 3. Add the unit to the lexer
+Text is turned into tokens (some of which are units) in `lexer.rs`. Here's one example:
+```rs
+// ...
+match string {
+  "h" | "hr" | "hrs" | "hour" | "hours" => tokens.push(Token::Unit(Hour)),
+  // etc
+}
+// ...
+```
+