Added in-using documentation.

Author: MairwunNx

documentation/in-using.md

@@ -0,0 +1,152 @@
+> ## Documentation for basically using CoreAPI.
+
+## 1. For playing and running minecraft:
+
+#### 1.1 Download Core API mod module.
+
+Visit **Core API** repository on github, visit **releases** tab and download `.jar` files of latest _pre-release_ / release (**recommended**)
+
+Releases page: https://github.com/ProjectEssentials/ProjectEssentials-Core/releases
+
+#### 1.2 Install Core API modification.
+
+The minecraft forge folder structure below will help you understand what is written below.
+
+```
+.
+├── assets
+├── config
+├── libraries
+├── mods (that's how it should be)
+│   └── Project Essentials Core-1.14.4-1.X.X.X.jar
+└── ...
+```
+
+Place your mods and Core API mods according to the structure above.
+
+#### 1.3 Verifying mod on the correct installation.
+
+Run the game, check the number of mods, if the list of mods contains `Project Essentials Core` mod, then the mod has successfully passed the initialization of the modification.
+
+## 2. For developing and developers:
+
+### 2.1 Getting started with installing.
+
+To get the Core API source for development and interactions with the rights of players, you need to get the dependencies and get the documentation to view it in your IDE.
+
+Installation documentation is located in the readme file or just follow the link: https://github.com/ProjectEssentials/ProjectEssentials-Core#-install-using-gradle
+
+### 2.2 API usage.
+
+I could not write this damn documentation at all and spend time on it, because it is so fucking understandable. But just in case, I will nevertheless sign here some trifles.
+
+Let's start small?
+
+#### 2.2.1 Functions.
+
+```
+EssBase.logBaseInfo
+
+- description: Print base modification information to log.
+```
+
+```
+EssBase.validateForgeVersion
+
+- description: Validate forge version on compatibility with loaded mod. If validation failed, then you will be notified with messages in logger with level WARN.
+```
+
+#### 2.2.2 Extensions.
+
+```
+CommandContext<CommandSource>.isPlayerSender
+
+- return: true if command sender is player. (boolean value)
+```
+
+```
+CommandEvent.commandName
+
+- example: player execute command `/heal MairwunNx`, then you get `heal` as string.
+
+- return: command name as string. (string value)
+```
+
+```
+CommandEvent.executedCommand
+
+- example: player execute command `/heal MairwunNx`, then you get `/heal MairwunNx` as string.
+
+- return: fully executed command as string. (string value)
+```
+
+```
+CommandEvent.player
+
+- return: `ServerPlayerEntity` class instance from `CommandEvent` class instance. (ServerPlayerEntity class instance)
+```
+
+```
+CommandEvent.source
+
+- return: Return command `source` from `CommandEvent` class instance. (CommandSource class instance)
+```
+
+```
+CommandSourceExtensions.sendMsg
+
+- description: Send localized message to player without logging.
+
+- accepts:
+    - moduleName - mod module name. (string)
+    - commandSource - command source instance. (CommandSource class instance)
+    - l10nString - localized string without `project_essentials_`. (string)
+    - args - additional arguments for localized string `(%s literals)`. (string arguments)
+```
+
+```
+String.capitalizeWords
+
+- return: capitalized each word string. (string)
+```
+
+```
+String.Companion.empty
+
+- return: empty string. (string)
+```
+
+#### 2.2.3 Helpers.
+
+```
+ForgePathHelper.getRootPath
+
+- accepts:
+    - pathType - enum class with CLIENT and SERVER enums. (ForgeRootPaths enum class)
+
+- return: absolutely path to configuration root dir. (string)
+```
+
+```
+ModErrorsHelper.<ERROR_ID>
+
+- see: https://github.com/MairwunNx/ProjectEssentials-Core/blob/c7e9c318efc78daeba68650f24036f379859f2a0/src/main/kotlin/com/mairwunnx/projectessentialscore/helpers/ModErrorsHelper.kt
+
+- return: error description and just error reason. (string)
+```
+
+```
+ModPathHelper.CONFIG_FOLDER
+
+- description: Minecraft config folder absolutely path.
+```
+
+```
+ModPathHelper.MOD_CONFIG_FOLDER
+
+- description: Project Essentials mod config folder.
+```
+
+## These are all API methods, I think you understand that everything is very simple.
+
+### For all questions, be sure to write issues!