add calculation to readme

README.md

@@ -4,11 +4,43 @@ Plot the bitcoin price and 200-week moving average.
 
 Please see [bitcoin-tools.github.io/btracker](https://bitcoin-tools.github.io/btracker/).
 
+## About the Math
+
+btracker is a calculation and data visualization tool that:
+- inputs historica price data
+- calculates the 200-week moving average
+- builds plots on linear and log scales
+- saves the calculated data to a CSV file
+- builds a static HTML page with the plots
+- deploys the HTML page [on GitHub Pages](https://bitcoin-tools.github.io/btracker)
+
+### Input Data
+
+The input data come from multiple sources.
+
+- For `2014-09-17` to `present`, the data are from [Yahoo Finance](https://finance.yahoo.com/quote/BTC-USD/history/).
+
+- For `2011-12-10` to `2014-09-16`, the data are from CoinMarketCap.
+
+Please open an issue to add a consistent source from pre-2011 through present.
+
+### Calculated Data - 200-Week Moving Average
+
+The 200-week moving average is the simple arithemtic mean of the Bitcoin price over the last 200 weeks.
+
+`btracker` uses the last 1,400 days, which improves granularity and accuracy over weekly data.
+
 ## Contributing
 
+This project is Open Source and welcomes contributions.
+
+### Report an Issue
+
 Please open an [issue](https://github.com/bitcoin-tools/btracker/issues) for any bug reports or feature requests.
 
-Pull requests are welcome.
+### Contribute Code
+
+Pull requests are welcome. For major changes, please open an issue first to discuss the implentation.
 
 ## License