Add Go gqlgen lib to list of federation support (#3706)

gqlgen - one of the most popular graphql Go lib - just recently added support for federation. 
PR to the lib was merged to master Jan 7 - 99designs/gqlgen#851

The lib doesn't have documentation for the feature yet, but it's fully working, as I am already using it in one of the projects.

docs/source/federation/other-servers.md

@@ -6,6 +6,7 @@ The following is a list of known open source GraphQL server libraries that have
 
 | Language    | Framework    | Library                                                                          |
 | ----------- | ------------ | -------------------------------------------------------------------------------- |
+| Go          | gqlgen       | [gqlgen](https://github.com/99designs/gqlgen/tree/master/plugin/federation)      |
 | Java        | graphql-java | [federation-jvm](https://github.com/apollographql/federation-jvm)                |
 | Kotlin      | graphql-java | [graphql-kotlin](https://github.com/ExpediaGroup/graphql-kotlin)                 |
 | PHP         | graphql-php  | [php-graphql-federation](https://github.com/pascaldevink/php-graphql-federation) |