add gortc/sdp (#2068)

Please check if what you want to add to `awesome-go` list meets [quality standards](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#quality-standard) before sending pull request. Thanks!

**Please provide package links to:**

- github.com repo: https://github.com/gortc/sdp
- godoc.org: https://godoc.org/github.com/gortc/sdp
- goreportcard.com: [![Go Report Card](https://goreportcard.com/badge/github.com/gortc/sdp)](https://goreportcard.com/report/github.com/gortc/sdp)
- coverage service link: [![codecov](https://codecov.io/gh/gortc/sdp/branch/master/graph/badge.svg)](https://codecov.io/gh/gortc/sdp)


**Note**: that new categories can be added only when there are 3 packages or more.

**Make sure that you've checked the boxes below before you submit PR:**
- [x] I have added my package in alphabetical order.
- [x] I have an appropriate description with correct grammar.
- [x] I know that this package was not listed before.
- [x] I have added godoc link to the repo and to my pull request.
- [x] I have added coverage service link to the repo and to my pull request.
- [x] I have added goreportcard link to the repo and to my pull request.
- [x] I have read [Contribution guidelines](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#contribution-guidelines), [maintainers note](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#maintainers) and [Quality standard](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#quality-standard).

Thanks for your PR, you're awesome! 👍

README.md

@@ -1271,6 +1271,7 @@ See [go-hardware](https://github.com/rakyll/go-hardware) for a comprehensive lis
     * [guesslanguage](https://github.com/endeveit/guesslanguage) - Functions to determine the natural language of a unicode text.
     * [inject](https://github.com/facebookgo/inject) - Package inject provides a reflect based injector.
     * [mxj](https://github.com/clbanning/mxj) - Encode / decode XML as JSON or map[string]interface{}; extract values with dot-notation paths and wildcards. Replaces x2j and j2x packages.
+    * [sdp](https://github.com/gortc/sdp) - SDP: Session Description Protocol [[RFC 4566](https://tools.ietf.org/html/rfc4566)].
     * [sh](https://github.com/mvdan/sh) - Shell parser and formatter.
     * [slug](https://github.com/gosimple/slug) - URL-friendly slugify with multiple languages support.
     * [Slugify](https://github.com/avelino/slugify) - Go slugify application that handles string.