Add a doc with all the current shortcuts (#785)

docs/shortcuts.md

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+### General
+
+| Function     | Mac | Other |
+| ------------ | :-: | :---: |
+| Load         |
+| Save         |
+| Export       |
+| Clear canvas |
+
+### Shapes
+
+| Function  |    Mac     |   Other    |
+| --------- | :--------: | :--------: |
+| Selection | `S` or `1` | `S` or `1` |
+| Rectangle | `R` or `2` | `R` or `2` |
+| Diamond   | `D` or `3` | `D` or `3` |
+| Ellipse   | `E` or `4` | `E` or `4` |
+| Arrow     | `A` or `5` | `A` or `5` |
+| Line      | `L` or `6` | `L` or `6` |
+| Text      | `T` or `7` | `T` or `7` |
+| Lock      |            |            |
+
+### Editor
+
+| Function       | Mac  |       Other        |
+| -------------- | :--: | :----------------: |
+| Copy           |  ⌘C  |     `Ctrl + C`     |
+| Paste          |  ⌘V  |     `Ctrl + V`     |
+| Copy styles    | ⇧⌘V  | `Ctrl + Shift + V` |
+| Paste styles   | ⇧⌘C  | `Ctrl + Shift + C` |
+| Delete         |  ⌫   |       `Del`        |
+| Send to back   | ⌥⌘\[ | `Ctrl + Shift + [` |
+| Bring to front | ⌥⌘\] | `Ctrl + Shift + ]` |
+| Send backwards | ⌘\[  |     `Ctrl + [`     |
+| Bring forward  | ⌘\]  |     `Ctrl + ]`     |
+
+### View
+
+| Function   | Mac |   Other    |
+| ---------- | :-: | :--------: |
+| Zoom in    | ⌘+  | `Ctrl + +` |
+| Zoom out   | ⌘-  | `Ctrl + -` |
+| Zoom reset | ⌘0  | `Ctrl + 0` |