added some notes about spreadsheet

spreadsheet.md

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+# Selectors
+
+`@[Col_Name]` - To get the column name
+
+`'SHEET_NAME'!$COL$ROW` - To get cell from given sheet name and specified row and col
+
+## Functions
+
+### `AVERAGE(number1, [number2], ...)`
+
+number1 - A number or cell reference that refers to numeric values.
+
+number2 - [optional] A number or cell reference that refers to numeric values.
+
+https://exceljet.net/excel-functions/excel-average-function
+
+### `COUNTIF(range, criteria)`
+
+range - The range of cells to count.
+
+criteria - The criteria that controls which cells should be counted.
+
+https://exceljet.net/excel-functions/excel-countif-function
+
+### `IF(condition, truthy_val, falsy_val)`
+
+condition - A value or condition that can evaluate to `TRUE` or `FALSE`
+
+truthy_val - The value to return when condition is `TRUE`
+
+falsy_val - The value too return when condition is `FALSE`
+
+https://exceljet.net/excel-functions/excel-if-function
+
+### `IFERROR(value, value_if_error)`
+
+value - The value, reference, or formula to check for an error.
+
+value_if_error - The value to return if an error is found.
+
+https://exceljet.net/excel-functions/excel-iferror-function
+
+### `MATCH (lookup_value, lookup_array, [match_type])`
+
+lookup_value - The value to match in lookup_array.
+
+lookup_array - A range of cells or an array reference.
+
+match_type - [optional] 1 = exact or next smallest (default), 0 = exact match, -1 = exact or next largest.
+
+https://exceljet.net/excel-functions/excel-match-function
+
+### `MIN (number1, [number2], ...)`
+
+number1 - Number, reference to numeric value, or range that contains numeric values.
+
+number2 - [optional] Number, reference to numeric value, or range that contains numeric values.
+
+https://exceljet.net/excel-functions/excel-min-function
+
+### `OFFSET(reference, rows, cols, [height], [width])`
+
+references - The starting point, supplied as a cell reference or range
+
+rows - The number of rows to offset below the starting reference.
+
+cols - The number of columns to offset to the right of the starting reference.
+
+height - [optional] The height in rows of the returned reference.
+
+width - [optional] The width in columns of the returned reference.
+
+https://exceljet.net/excel-functions/excel-offset-function