Add SDK readme

sdk/README.md

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+<p>
+  <a href="http://www.gwos.com/" target="blank"><img src="../.github/img/readme_image.png" alt="GWOS Logo"/></a>
+</p>
+
+[![License](https://img.shields.io/github/license/gwos/tcg)](LICENSE)
+[![GoDoc](https://godoc.org/github.com/gwos/tcg/sdk?status.svg)](https://godoc.org/github.com/gwos/tcg/sdk)
+[![Go Report Card](https://goreportcard.com/badge/github.com/gwos/tcg/sdk)](https://goreportcard.com/report/github.com/gwos/tcg/sdk)
+
+# TCG SDK
+
+SDK provides data structures, clients, and featured packages for integration with Groundwork Monitoring.
+
+Known integrations:
+- [TCG itself](https://github.com/gwos/tcg)
+- [Telegraf output plugin](https://pkg.go.dev/github.com/influxdata/telegraf/plugins/outputs/groundwork)
+- Diamanti Picasa collector
+
+
+<a name="dependencies"></a>
+## Dependencies
+
+No 3rd party dependencies.
+Only standard library imports allowed in this module to simplify integrations and prevent conflicts of dependencies with different versions.
+
+
+<a name="envvar"></a>
+## Environment variables
+
+SDK HTTP clients processed environment variables:
+- `TCG_TLS_CLIENT_INSECURE` [InsecureSkipVerify in tls.Config](https://pkg.go.dev/crypto/tls#Config), __false__ by default.
+- `TCG_HTTP_CLIENT_TIMEOUT` [Timeout in http.Client](https://pkg.go.dev/net/http#Client), __5s__ by default.
+- `TCG_HTTP_CLIENT_TIMEOUT_GW`, same as `TCG_HTTP_CLIENT_TIMEOUT` on communication with Groundwork Foundation server, __40s__ by default.
+
+
+<a name="logging"></a>
+## Logging
+
+SDK module uses [log/slog](https://pkg.go.dev/log/slog) based logger. In oder to capture it [implement custom Handler](https://pkg.go.dev/log/slog#hdr-Writing_a_handler) and assign to module [Logger](https://pkg.go.dev/github.com/gwos/tcg/sdk/log#pkg-variables).
+
+For example, see logger adapter in [Telegraf output plugin](https://pkg.go.dev/github.com/influxdata/telegraf/plugins/outputs/groundwork).