Merge pull request NuGet#2151 from NuGet/master

Added 5.7 release notes to TOC

Author: Christopher R. Gill

docs/TOC.md

@@ -290,6 +290,7 @@
 ## Release notes
 ### [Known Issues](release-notes/known-issues.md)
 ### NuGet 5.x
+#### [NuGet 5.7](release-notes/NuGet-5.7.md)
 #### [NuGet 5.6](release-notes/NuGet-5.6.md)
 #### [NuGet 5.5](release-notes/NuGet-5.5.md)
 #### [NuGet 5.4](release-notes/NuGet-5.4.md)