BUG#36381815 step 3.1: Update readme files after previous step

Part of: Bug#36381815: Factor out generic functionality from the mysql_binlog_event library

Update libs/mysql/readme.md to list the new library and describe the
library in libs/mysql/allocators/readme.md.

(We don't squash this into step 2, in order to keep step 2
reproducible with scripts.)

Change-Id: I71ecefd931401517718a5a3c16f384a4826255c5

Author: Sven Sandberg

libs/mysql/allocators/readme.md

@@ -31,3 +31,30 @@ MySQL Allocators Library
 -->
 
 Code documentation: @ref GroupLibsMysqlAllocators.
+
+## Summary
+
+Allows libraries to allocate memory such that:
+- The server can use the library in such a way that the memory is
+  performance_schema-instrumented.
+- Other code can use the library without performance_schema instrumentation.
+- The library is never dependent on performance_schema.
+
+## Usage
+
+This library provides the `Memory_resource` class, which is used in different
+ways depending on who you are:
+
+- *Library author*: If your library needs to allocate memory, add a
+  `Memory_resource` parameter to either the function that allocates memory or
+  the constructor for the class that manages the memory. The parameter must be
+  passed by value, not reference or pointer. Allocate memory through this
+  object. Make the parameter default to `Memory_resource()`.
+
+- *Library user within the MySQL server*: When using the function or class that
+  takes a `Memory_resource` object, call `psi_memory_resource(Psi_memory_key)`
+  to get the `Memory_resource` object to pass to the function or constructor.
+
+- *Library user outside the MySQL server*: When using the function or class
+  constructor that takes an optional `Memory_resource` parameter, allow that
+  parameter to take its default value.

libs/mysql/readme.md

@@ -37,8 +37,9 @@ which is subject to change. Use on your own responsibility.
 For now, we build only the static libraries.
 
 Libraries defined on this level:
-- \subpage PageLibsMysqlGtid
+- \subpage PageLibsMysqlAllocators
 - \subpage PageLibsMysqlBinlog
+- \subpage PageLibsMysqlGtid
 - \subpage PageLibsMysqlSerialization
 - \subpage PageLibsMysqlUtils