Merge pull request #375 from mcarmonaa/improvement/docs-indexes

docs: add indexes section

Author: ajnavarro

docs/using-gitbase/indexes.md

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+# Indexes
+
+`gitbase` allows you to speed up queries creating indexes.
+
+Indexes are implemented as bitmaps using [pilosa](https://github.com/pilosa/pilosa) as a backend storage for them. To run a pilosa instance see the [getting started](./getting-started.md) section.
+
+Thus, to create indexes you must specify pilosa as the type of index. You can find some examples in the [examples](./examples.md#create-an-index-for-columns-on-a-table) section about managing indexes.
+
+Note that you can create an index either **on one or more columns** or **on a single expression**.
+
+You can find some more examples in the [examples](./examples.md#create-an-index-for-columns-on-a-table) section.
+
+See [go-mysql-server](https://github.com/src-d/go-mysql-server/tree/e03c0c8264d306f683a8c1e7933c5d2f4b200a5b#indexes) documentation for more details