Create system-database.md (pingcap#201)

* Create system-database.md

* Address comments

* Update format

* Add description

* Add link for system database

sql/README.md

@@ -12,7 +12,7 @@ TiDB supports the SQL-92 standard and is compatible with MySQL. To help you easi
 + TiDB Server Administration
     - The TiDB Server
     - The TiDB Data Directory
-    - The TiDB System Database
+    - [The TiDB System Database](system-database.md)
     - [The Proprietary System Variables and Syntax in TiDB](tidb-specific.md)
     - TiDB Server Logs
 + Security

sql/system-database.md

@@ -0,0 +1,36 @@
+---
+title: The TiDB System Database
+category: user guide
+---
+
+# The TiDB System Database
+
+The TiDB System Database is similar to MySQL, which contains tables that store information required by the server when it runs.
+
+## Grant System Tables
+
+These system tables contain grant information about user accounts and their privileges:
+
+- `user`: user accounts, global privileges, and other non-privilege columns
+- `db`: database-level privileges
+- `tables_priv`: table-level privileges
+- `columns_priv`: column-level privileges
+
+## Server-Side Help System Tables
+
+Currently, the `help_topic` is NULL.
+
+## Statistics System Tables
+
+- `stats_buckets`: the buckets of statistics
+- `stats_histograms`: the histograms of statistics
+- `stats_meta`: the meta information of tables, such as the total number of rows and updated rows
+
+## GC Worker System Tables
+
+- `gc_delete_range`: to record the data to be deleted
+
+## Miscellaneous System Tables
+
+- `GLOBAL_VARIABLES`: global system variable table
+- `tidb`: to record the version information when TiDB executes `bootstrap`