Add documentation on references

DOCS.md

@@ -8,21 +8,21 @@ around your calculations, so that they will make sense later.
 $500 for tickets + $70 for a taxi
 ```
 
-# Formatting
+## Formatting
 
 Use `#` symbol at beginning of a line to mark it as a **header**.
 
 Use `:` colon to mark first part of line as **description**.
 
-# Numbers
+## Numbers
 
 - `0.5 + .5` - decimal numbers
 - `1k` - one thousand
 - `1 000 000` - million with spaces
 - `1,000,000` - million with commas
 - `1M` - million with letter
 
-# Operators
+## Operators
 
 Supported operators `+`, `-`, `*` or `x`, `/`, `%`, `^`.
 
@@ -32,7 +32,7 @@ Supported operators `+`, `-`, `*` or `x`, `/`, `%`, `^`.
 - Percentage subtraction `100 - 25%`
 - Percentage calculation `25% of 100` or `100 * 25%`
 
-# Currencies
+## Currencies
 
 Numbr can do math with lots of currencies. Currency exchange rates are updated every hour.
 
@@ -47,26 +47,38 @@ Numbr supports cryptocurrencies as well:
 - `1 eth in doge`
 - `10 xrp in ada`
 
-# Sum
+## Sum
 
 Numbr can summarize results by using `total` or `sum` operators. Calculations are done up to the nearest header:
 
 ```md
 ## header 1
+
 Milk $3
 Eggs $2
 Sugar $1
 Flour $5
 Total
 
 ## header 2
+
 100k usd
 10 btc
 1M chf
 Sum
 ```
 
-# Variables
+## References
+
+Numbr supports referencing calculation results. Click on the answer to copy, and paste to reference, or press
+*Alt+Enter* to put the current answer on the next line.
+
+```
+100 + 200                 | 300
+(300) + 25%               | 375
+```
+
+## Variables
 
 Numbr supports variables with `=` operator. Variable names are case-insensitive and can contain letters, numbers and
 spaces: