Add READMEs to pkg packages (#1352)

Signed-off-by: Darko Draskovic <darko.draskovic@gmail.com>

pkg/README.md

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+# Standalone packages
+
+The `pkg` directory (the current directory) contains a set of standalone packages that can be imported and used by external applications. The packages are specifically meant for the development of the Mainflux based back-end applications and implement common tasks needed by the programmatic operation of Mainflux platform.

pkg/auth/README.md

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+# Auth client
+
+Auth client package is used to identify a thing and authorize a thing's access to a channel.
+
+To identify a thing, you need a valid **thing key**. You retrieve thing's identity in the form of a **thing ID**. The latter is used in CRUD operations on things and their connections.
+
+To authorize a thing's access to a channel, you need a valid **thing ID** and a valid **channel ID**. If a thing is not connected to a channel, the auth client responds with an error. Otherwise, a *nil* value is returned, signaling the successful authorization.

pkg/errors/README.md

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+# Errors
+
+`errors` package serve to build an arbitrary long error chain in order to capture errors returned from nested service calls.
+
+`errors` package contains the custom Go `error` interface implementation, `Error`. You use the `Error` interface to **wrap** two errors in a containing error as well as to test recursively if a given error **contains** some other error.

pkg/messaging/README.md

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+# Messaging
+
+`messaging` package defines `Publisher`, `Subscriber` and an aggregate `Pubsub` interface. 
+
+`Subscriber` interface defines methods used to subscribe to a message broker such as MQTT or NATS. 
+
+`Publisher` interface defines methods used to publish messages to a message broker such as MQTT or NATS.
+
+`Pubsub` interface is composed of `Publisher` and `Subscriber` interface and can be used to send messages to as well as to receive messages from a message broker.

pkg/sdk/README.md

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+# Mainflux SDK kits
+
+This directory contains drivers for Mainflux HTTP API. Drivers facilitate system administration - CRUD operations on things, channels and their connections, i.e. provision of Mainflux entities. They can be used also for messaging. 
+
+Drivers are written in different languages in order to enable the faster application development in the respective language.

pkg/transformers/README.md

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 # Message Transformers
 
-Transformers services consume events published by adapters and transform them to any other message format.
-They be imported as a standalone package and used for message transformation on the consumer side.
+A transformer service consumes events published by Mainflux adapters (such as MQTT and HTTP adapters) and transforms them to an arbitrary message format. A transformer can be imported as a standalone package and used for message transformation on the consumer side.
+
 Mainflux [SenML transformer](transformer) is an example of Transformer service for SenML messages.
+
 Mainflux [writers](writers) are using a standalone SenML transformer to preprocess messages before storing them.
 
 [transformers]: https://github.com/mainflux/mainflux/tree/master/transformers/senml

pkg/ulid/README.md

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+# ULID identity provider
+
+ULID identity provider generates a universally unique lexicographically sortable, string encoded identifier, a 128-bit number, unique for all practical purposes.

pkg/uuid/README.md

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+# UUID identity provider
+
+The UUID identity provider generates a random, universally unique identifier (UUID), unique for all practical purposes.