Update documentation

Author: harish2704

README.md

@@ -1,2 +1,48 @@
 # simple-db-migrator
 A simple database migration tool written in PHP
+
+# Quick introduction.
+
+1. This is a simple database migration commandline application written in php.
+2. It has following dependencies. 
+    * `symfony/console` for argument parsing ( of cli interface )
+    * `psr/log` for managing and priting log messages.
+3. Contents of each SQL file is run as single transaction.
+4. The migrator tool will save the `down` migration (rollback SQL statement) in DB and cross verify it with the current version of the rollback SQL statement present in the disk and complain if both are different. 
+
+# Usage
+
+Migrations are arranged in the following directory structure.
+```
+├── migrations
+│   ├── down
+│   │   ├── 001.sql
+│   │   ├── 002.sql
+│   │   ├── 003.sql
+│   │   └── 004.sql
+│   └── up
+│       ├── 001.sql
+│       ├── 002.sql
+│       ├── 003.sql
+│       └── 004.sql
+└── simple-db-migrator.php
+
+```
+
+To create a new migration, Just create respective `xxx.sql` file in `up` and `down` directories.
+
+## Run initial migration ( ie, create db_migration table )
+```bash
+# '-s' option stands for 'setup'
+php simple-db-migrator.php -s
+```
+
+## Apply all pending migrations ( up )
+```bash
+php simple-db-migrator.php -u
+```
+
+## Rollback last migration ( down )
+```bash
+php simple-db-migrator.php -d
+```