Mapname.txt

You can ship the file <game>/maps/<mapname>.txt with your map file to display custom text for that map.

Counter-Strike: Source

The plain text in the file cstrike/maps/cs_assault.txt is shown before players choose a team.

The text in that file is displayed as plain text in the UI when choosing a team. See the screenshot on the right.

Counter-Strike: Global Offensive

An edited loading screen. The content of the file csgo/maps/de_testingmap.txt is:

RULESHERE

<font color='#727272'>This is an example for the Valve Developer Community</font><font color='#ff9844'> Colored Text! </font> 

The text in that file is displayed in the loading screen. The HTML tag <font color='#FFFFFF'>...</font> can be used for colors. Also, the following formats can be used:

CS:GO Loading screen texts (de_ map, classic game mode)

<mapname>.txt content	Result (English)
COMMUNITYMAPCREDITS: any other text	Bomb Scenario Mission #SFUI_Rules_<gamemode>_Loading A community map created by: any other text
RULESHERE any other text	Bomb Scenario Mission #SFUI_Rules_<gamemode>_Loading any other text
any other text	Loading...any other text
none	Loading...YOUR TEXT FILE IS BLANK! If you intend for this space to be blank, open up '<mapname>.txt', enter a single space and save the file.

Note:

• The #SFUI_Rules_<gamemode>_Loading mentioned in the table is actually a language and game mode specific string which is looked up from the text file csgo/resource/csgo_<language>.txt. On the screenshot on the right it is the string "Buy new weapons [...] Best out of 15 rounds".
• The text "Bomb Scenario Mission" is game mode specific and generated from the map prefix "de_". Maps with "cs_" instead generate "Hostage Scenario Mission" and all other map names deliver "Elimination Mission", no matter the actual scenario!
• The key word COMMUNITYMAPCREDITS: or RULESHERE must be the first word of the text file to deliver the above result!