update links in docs (#4010)

Author: vitek-rostislav

CONTRIBUTING.md

@@ -7,4 +7,4 @@ This repository is READ-ONLY.
 If you want to [report issues](https://github.com/shopsys/shopsys/issues/new) and/or send [pull requests](https://github.com/shopsys/shopsys/compare),
 please use the main [Shopsys repository](https://github.com/shopsys/shopsys).
 
-Please, check our [Contribution Guide](https://github.com/shopsys/shopsys/blob/master/CONTRIBUTING.md) before contributing.
+Please check our [Contribution Guide](https://github.com/shopsys/shopsys/blob/HEAD/CONTRIBUTING.md) before contributing.

README.md

@@ -4,7 +4,7 @@
 
 This bundle for [Shopsys Platform](https://www.shopsys.com) adds frontend API using [overblog/GraphQLBundle](https://github.com/overblog/GraphQLBundle).
 The bundle is dedicated for projects based on Shopsys Platform (i.e. created from [`shopsys/project-base`](https://github.com/shopsys/project-base)) exclusively.
-This repository is maintained by [shopsys/shopsys] monorepo, information about changes is in [monorepo CHANGELOG.md](https://github.com/shopsys/shopsys/blob/master/CHANGELOG.md).
+This repository is maintained by [shopsys/shopsys] monorepo, information about changes is in its `CHANGELOG` file.
 
 ## Documentation
 
@@ -35,7 +35,7 @@ This repository is READ-ONLY.
 If you want to [report issues](https://github.com/shopsys/shopsys/issues/new) and/or send [pull requests](https://github.com/shopsys/shopsys/compare),
 please use the main [Shopsys repository](https://github.com/shopsys/shopsys).
 
-Please, check our [Contribution Guide](https://github.com/shopsys/shopsys/blob/master/CONTRIBUTING.md) before contributing.
+Please check our [Contribution Guide](https://github.com/shopsys/shopsys/blob/HEAD/CONTRIBUTING.md) before contributing.
 
 ## Support