Added docs and test to confirm "" in branch config will override default

Fixes #692

Author: JakeGinnivan

docs/configuration.md

@@ -66,7 +66,8 @@ The options in here are:
  - **`mode:`** Same as above
 
  - **`tag:`** The pre release tag to use for this branch. Use the value `use-branch-name-as-tag` to use the branch name instead.  
-   For example `feature/foo` would become a pre-release tag of `foo` with this value
+   For example `feature/foo` would become a pre-release tag of `foo` with this value  
+   **Note:** To clear a default use an empty string: `tag: ""`
 
  - **`increment:`** the part of the SemVer to increment when GitVersion detects it needs to be (i.e commit after a tag)
 

src/GitVersionCore.Tests/ConfigProviderTests.cs

@@ -88,6 +88,21 @@ public void OverwritesDefaultsWithProvidedConfig()
         config.Branches["dev(elop)?(ment)?$"].Tag.ShouldBe("dev");
     }
 
+    [Test]
+    public void CanRemoveTag()
+    {
+        const string text = @"
+next-version: 2.0.0
+branches:
+    releases?[/-]:
+        tag: """"";
+        SetupConfigFileContent(text);
+        var config = ConfigurationProvider.Provide(repoPath, fileSystem);
+
+        config.NextVersion.ShouldBe("2.0.0");
+        config.Branches["releases?[/-]"].Tag.ShouldBe(string.Empty);
+    }
+
     [Test]
     public void CanProvideConfigForNewBranch()
     {