Create basic Keywords page

Keywords.md

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+# Keywords
+
+EssentialsX provides a number of standard keywords (placeholders) that can be used to customize text in multiple places across the plugin.
+
+| Keyword                 | Usage                                                        |
+| ----------------------- | ------------------------------------------------------------ |
+| {PLAYER}, {DISPLAYNAME} | The player's display name.                                   |
+| {USERNAME}              | The player's username.                                      |
+| {NICKNAME}              | The player's EssentialsX nickname. If the player has no nickname, the username is used. |
+| {PREFIX}                | The player's prefix, supplied by a permissions plugin via Vault. |
+| {SUFFIX}                | The player's suffix, supplied by a permissions plugin via Vault. |
+| {GROUP}                 | The player's primary group, supplied by a permissions plugin via Vault. |
+| {BALANCE}               | The player's balance, formatted as currency.                 |
+| {MAILS}                 | The amount of mail the player has.                           |
+| {PLAYTIME}              | The player's playtime, formatted as a duration.              |
+| {WORLD}, {WORLDNAME}    | The full name of the player's current world.                 |
+| {WORLDTIME12}           | The time of the player's current world in AM/PM format.      |
+| {WORLDTIME24}           | The time of the player's current world in 24-hour format.    |
+| {WORLDDATE}             | The date of the player's current world.                      |
+| {IP}                    | The player's IP address.                                     |
+| {ADDRESS}               | The player's socket address.                                 |
+| {ONLINE}                | The amount of players online. Hidden players (e.g. those in vanish) are not counted. |
+| {UNIQUE}                | The amount of unique players that have joined the server.    |
+| {WORLDS}                | The list of worlds on the server.                            |
+| {PLAYERLIST}            | The list of players currently online. Hidden players (e.g. those in vanish) are not listed. |
+| {TIME}                  | The current time of the machine the server is running on.    |
+| {DATE}                  | The current date of the machine the server is running on.    |
+| {TPS}                   | The server's current TPS.                                    |
+| {UPTIME}                | The time elapsed since the server started up, formatted as a duration. |
+| {PLUGINS}               | The list of plugins on the server.                           |
+| {VERSION}               | The server's version string.                                 |
+
+### Where can I use these keywords?
+
+These keywords can be used with the following EssentialsX features:
+
+- Custom text files: `book.txt`, `custom.txt`, `info.txt`, `motd.txt`, `rules.txt`
+- Kits: inside the `kits.yml` file.
+- EssentialsX Spawn's newbie announcement: `newbies.announce-format` in the config.
+- The `/broadcastworld` command.
+