Add more README.md for calculator, currency, minesweeper.

currency/README.md

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+# Doughnut — An exchange rate tracker for people nuts about dough, in PyQt.
+
+This is a simple currency exchange rate tracker implemented in PyQt, using the [fixer.io](http://fixer.io) API
+for data. The default setup shows currency data for the preceding 180 days.
+
+![Doughnut](screenshot-currency1.png)
+
+Data is loaded progressively, with increasing resolution. Currency rates for a given data are shown in the right
+hand panel and updated to follow the position of the mouse.
+
+![Doughnut](screenshot-currency2.png)
+
+> If you think this example app is neat and want to learn more about
+PyQt in general, [take a look at my ebook & online course
+"Create Simple GUI Applications"](https://martinfitzpatrick.name/create-simple-gui-applications)
+which covers everything you need to know to start building your own applications with PyQt.
+
+## Code notes
+
+# Data handling
+
+The interface presents a tracking plot (using PyQtGraph) of rates over the past 180 days. Since we don't want to 
+spam a free service, requests to the API are rate-limited to 1-per-second, giving a full-data-load time of 180s (3 min).
+
+To avoid waiting each time, we use `requests_cache` which uses a local sqlite database to store the result of recent
+requests. The requests for data use a progressive 'search' approach: where there is a gap in the data, the middle 
+point is filled first, and it prefers to load the most recent timepoints first. This means the whole plot gradually
+increases in resolution over time, rather than working backwards only.
+
+# Conversions
+
+By default the app retrieves EUR rates and shows conversions to this base currency. If you change base currency
+it will retrieve all data again for that new currency. This is daft, since if we have rates vs. EUR we can calculate
+any other currency->currency conversion via EUR (with a potential loss of accuracy).
+
+