Add TypeScript details to READMEs

And in some cases, add READMEs

(this is a follow-up to #2589)

apps/btadv/README.md

@@ -0,0 +1,16 @@
+# Bluetooth Advert
+
+This app advertises and exports (over Bluetooth) live data from the bangle's sensors:
+
+- Heart Rate
+- Accelerometer readings
+- Pressure
+- GPS information
+- Magnetic flux
+
+Swipe in any direction to access settings, and tap a setting to toggle it.
+Hit back to return to the details screen, which shows sensor data being exported.
+
+# TypeScript
+
+This app is written in TypeScript, see [typescript/README.md](/typescript/README.md) for more info

apps/clkinfostopw/README.md

@@ -9,3 +9,7 @@ Tap to start, tap again to pause. Tap again to restart
 ## Requests
 
 [Contact Rob](https://www.github.com/bobrippling) for features/bugs
+
+# TypeScript
+
+This app is written in TypeScript, see [typescript/README.md](/typescript/README.md) for more info

apps/widChargingStatus/README.md

@@ -0,0 +1,8 @@
+# Charging Status
+
+This widget shows a yellow lightning icon to indicate that the watch is charging. A short buzz is also emitted on initial charging connection.
+Nothing is shown when not charging.
+
+# TypeScript
+
+This app is written in TypeScript, see [typescript/README.md](/typescript/README.md) for more info