Update readme

README.md

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 # pdfjs
-Electron app for viewing PDF with simple vim binding
+Electron app for viewing PDF with simple vim binding.
+
+Using:
+- Electron https://github.com/electron/electron
+- PDF.js https://mozilla.github.io/pdf.js/
+
+Update keyboard shortcuts:
+- PDF.js support many keyboard shortcuts https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#what-are-the-pdfjs-keyboard-shortcuts, but unfortunately does not support j/k to move up/down.
+- Switch old `j`/`k` (page down/page up) to `d/u`
+- Implement new keybind `j`/`k` to move `down`/`up`
+
+# Install
+
+```
+npm install
+
+npm start
+```