You need a few icon files as well, then copy and paste the following into an xml file (CityPark.xml) in the Mods folder. Note that I added the basic cost of 10 gold and 5 mats, you can remove that if you don't want.
<ImprovementTypes>
<ImprovementType InternalName="K_CityPark">
<DisplayName>City Park</DisplayName>
<Description>City parks increase the Prestige produced by the settlement by 1.</Description>
<TileDesign>K_CityPark</TileDesign>
<ConstructionTileDesign>K_CityPark_UC</ConstructionTileDesign>
<DrawnIcon>Gfx/TacticalIcons/K_CityPark.png</DrawnIcon>
<DrawnIconConstruction>Gfx/TacticalIcons/K_CityPark_UC.png</DrawnIconConstruction>
<Thumbnail>K_CityPark_Thumb.png</Thumbnail>
<AllowedPerCity>1</AllowedPerCity>
<ReqCityLevel>4</ReqCityLevel>
<LaborToBuild>10.0</LaborToBuild>
<SupportedTerrainType>Land</SupportedTerrainType>
<SupportedTerrainType>City</SupportedTerrainType>
<RequiresCity>True</RequiresCity>
<Medallions>
<All>K_CityPark_Thumb.png</All>
<Border_All>
</Border_All>
</Medallions>
<Prereq>
<Type>Tech</Type>
<Attribute>Civics_Amarian</Attribute>
<Value>0</Value>
</Prereq>
<Prereq>
<Type>Allegiance</Type>
<Attribute>Kingdom</Attribute>
</Prereq>
<GameModifier InternalName="Prestige1">
<ModType>Resource</ModType>
<Attribute>Prestige</Attribute>
<Value>1.0</Value>
<PerTurn>1</PerTurn>
</GameModifier>
<GameModifier>
<ModType>ConstructionResourceCost</ModType>
<Attribute>Gold</Attribute>
<Value>-10</Value>
</GameModifier>
<GameModifier>
<ModType>ConstructionResourceCost</ModType>
<Attribute>Materials</Attribute>
<Value>-5</Value>
</GameModifier>
<GameModifier>
<ModType>Resource</ModType>
<Attribute>Gold</Attribute>
<Value>-1.0</Value>
<PerTurn>1</PerTurn>
</GameModifier>
<AIData AIPersonality="AI_General">
<AITag>Prestige</AITag>
</AIData>
</ImprovementType>
</ImprovementTypes>
Untested, but should work. Enjoy.