Add command front end reference to readme.md

Author: KirillMysnik

readme.md

@@ -2,3 +2,42 @@
 
 ## Introduction
 Source.Python Admin is an open-source project that uses the [Source.Python](https://github.com/Source-Python-Dev-Team/Source.Python) framework to allow for easy Administration on Source-engine servers.
+
+## Front Ends
+### Command Front End
+Command front end allows executing features through chat and client commands.
+#### Syntax:
+- `!spa <command and sub-commands>` - public chat command
+- `/spa <command and sub-commands>` - private chat command
+- `spa <command and sub-commands>` - client command
+
+The difference between public and private chat commands is that the private one suppresses the command string, so that nobody receives a chat message containing it.
+Client commands are run via player's game console.
+
+Player-based features (those have some kind of a player target) can also be executed via command front end.
+#### Syntax (`!spa`, `/spa` and `spa` prefixes are omitted here, but otherwise are required):
+`<command and sub-commands> <player filter> <filter argument>`
+
+Where *player filter* is one of the following:
+- `name` - this filter targets player by their name; *filter argument* is required in this case.
+- `steamid`, `steam` - this filter targets player by their SteamID; *filter argument* is required in this case.
+- `index` - this filter targets player by their entity index; *filter argument* is required in this case.
+- `@<built-in template filter>` - this filter targets player using one of the template player filters; *filter argument* is omitted in this case; *built-in template filter* is one of the following:
+  - `me`, `self` - targets the command issuer.
+  - `all` - targets all players on the server.
+  - `bot` - targets all bots on the server (including GOTV/SourceTV).
+  - `human` - targets all human players on the server; opposite to `bot`.
+  - `dead` - targets all dead players on the server.
+  - `alive` - targets all alive players on the server; opposite to `dead`.
+  - `un` - targets all unassigned players (those who didn't select their team).
+  - `spec` - targets all spectators.
+  - `t` - targets all team #2 players (terrorists, rebels, RED team etc).
+  - `ct` - targets all team #3 players (counter-terrorists, combines, BLU team etc).
+  - depending on the game, additional team filters might be defined.
+- `#<userid>` - this filter targets player by their UserID (as reported by the `status` console command); *filter argument* is omitted in this case; *userid* is the target's UserID.
+- `!<inverted player filter>` - this filter targets all players, but only not those who would otherwise be targeted by *inverted player filter*; *filter argument* is passed to the inverted player filter.
+
+#### Examples:
+- `/spa slay @all` - slays all players on the server
+- `/spa slay !@me` - slays all players except the player who issues the command
+- `/spa resurrect name iPlayer` - resurrects the player whose name matches "iPlayer"