Add TT_PRELOAD doc

doc/book/admin/instance_config.rst

@@ -1,8 +1,7 @@
 .. _admin-instance_config:
 
-================================================================================
 Instance configuration
-================================================================================
+======================
 
 For each Tarantool instance, you need two files:
 
@@ -50,9 +49,8 @@ For each Tarantool instance, you need two files:
 
 .. _admin-instance_file:
 
---------------------------------------------------------------------------------
 Instance file
---------------------------------------------------------------------------------
+-------------
 
 After this short introduction, you may wonder what an instance file is, what it
 is for, and how ``tarantoolctl`` uses it. After all, Tarantool is an application
@@ -84,11 +82,45 @@ entire application business logic in it. We, however, do not recommend this,
 since it clutters the instance file and leads to unnecessary copy-paste when
 you need to run multiple instances of an application.
 
+.. _admin-tt-preload:
+
+Preloading Lua scripts and modules
+----------------------------------
+
+Tarantool supports loading and running chunks of Lua code before the loading instance file.
+To load or run Lua code immediately upon Tarantool startup, specify the ``TT_PRELOAD``
+environment variable. Its value can be either a path to a Lua script, or a Lua module name.
+
+*   Run the Lua script ``script.lua`` from the ``preload/path/`` directory inside
+    the working directory in Tarantool before executing ``main.lua``:
+
+    .. code-block:: bash
+
+        TT_PRELOAD=/preload/path/script.lua tarantool main.lua
+
+*   Load the ``preload.module`` into the Tarantool Lua interpreter
+    executing ``main.lua``:
+
+    .. code-block:: bash
+
+        TT_PRELOAD=preload.module tarantool main.lua
+
+    .. warning::
+
+        ``TT_PRELOAD`` values that end with ``.lua`` are considered scripts,
+        so avoid modules names with this ending.
+
+To load several scripts or modules, pass them in a single quoted string separated
+by semicolons:
+
+.. code-block:: bash
+
+    TT_PRELOAD="/preload/path/script.lua;preload.module" tarantool main.lua
+
 .. _admin-tarantoolctl_config_file:
 
---------------------------------------------------------------------------------
 tarantoolctl configuration file
---------------------------------------------------------------------------------
+-------------------------------
 
 While instance files contain instance configuration, the ``tarantoolctl``
 configuration file contains the configuration that ``tarantoolctl`` uses to