Move breaking changes doc to /docs

docs/tutorial/planned-breaking-changes.md

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+# Planned Breaking API Changes
+
+The following list includes the APIs that will be removed in Electron 2.0.
+
+There is no timetable for when this release will occur but deprecation
+warnings will be added at least 90 days beforehand.
+
+```js
+// Deprecated
+clipboard.readRtf()
+// Replace with
+clipboard.readRTF()
+
+// Deprecated
+clipboard.writeRtf()
+// Replace with
+clipboard.writeRTF()
+
+// Deprecated
+clipboard.readHtml()
+// Replace with
+clipboard.readHTML()
+
+// Deprecated
+clipboard.writeHtml()
+// Replace with
+clipboard.writeHTML()
+
+// Deprecated
+nativeImage.toPng()
+// Replace with
+nativeImage.toPNG()
+
+// Deprecated
+nativeImage.toJpeg()
+// Replace with
+nativeImage.toJPEG()
+
+// Deprecated
+new BrowserWindow({webPreferences: {blinkFeatures: ''}})
+// Replace with
+new BrowserWindow({webPreferences: {enableBlinkFeatures: ''}})
+
+// Deprecated
+tray.setHighlightMode(true)
+// Replace with
+tray.setHighlightMode('on')
+
+// Deprecated
+tray.setHighlightMode(false)
+// Replace with
+tray.setHighlightMode('off')
+
+// Deprecated
+webContents.openDevTools({detach: true})
+// Replace with
+webContents.openDevTools({mode: 'detach'})
+```