added indexes introduction

manual/docs/Getting started - Indexes - Introduction.php

+Indexes are used to find rows with specific column values quickly. 
+Without an index, the database must begin with the first row and then read through the entire table to find the relevant rows. 
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+The larger the table, the more this consumes time. If the table has an index for the columns in question, the database 
+can quickly determine the position to seek to in the middle of the data file without having to look at all the data. 
+If a table has 1,000 rows, this is at least 100 times faster than reading rows one-by-one.
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+You should *<b>always</b>* use indexes for the fields that are used in sql where conditions.