Update README.md with command information (#81)

It wasn't immediately obvious from the README how to access the
commandline or what commands are supported. This changeset adds a brief
overview of the command system and lists current and future commands
based off the command tracking issue [1] at the time of writing.

[1] #70

README.md

@@ -52,6 +52,40 @@ For now, there are only two shortcuts:
 - `^w` saves the current view
 - `^c` exits
 
+## Commands
+`xi-term` supports a vim-like command prompt for executing commands, accessed
+via Alt-X.
+
+Currently supported commands:
+
+| Short form | Long form | Description |
+| ---------- | --------- | ----------- |
+| q | quit | Quits xi-term |
+| s | save | Saves the current file |
+| o `filename` | open `filename` | Open `filename` for editing |
+| bn | next-buffer | Switch to the next buffer |
+| bp | prev-buffer | Switch to the previous buffer |
+| t `theme` | theme `theme-name` | Set the theme to `theme`|
+
+Future commands:
+
+| Short form | Long form | Description |
+| ---------- | --------- | ----------- |
+ | c | close | Closes the current view |
+ | b | backspace | Delete the previous character and move the cursor one position back |
+ | d | delete | Delete the character under the cursor |
+ | ? `string` | search `string` | Search for `string` |
+ | pd | page-down | Advance the current view by one page |
+ | pu | page-up | Move the current view back by one page |
+ | ml | move-left | Move the cursor one position left |
+ | mr | move-right | Move the cursor one position right |
+ | mu | move-up | Move the cursor one line up |
+ | md | move-down | Move the cursor one line down |
+ | sl | select-left | Move the cursor one position left and update the current selection accordingly |
+ | sr | select-right | Move the cursor one position right and update the current selection accordingly |
+ | su | select-up | Move the cursor one line up and update the current selection accordingly |
+ | sd | select-down | Move the cursor one line down and update the current selection accordingly |
+
 ## Caveats
 
 ### Tabs