README: include build badge for v0.28 builds

Include a build badge for `maint/v0.28` builds.

Author: ethomson

README.md

@@ -4,6 +4,7 @@ libgit2 - the Git linkable library
 | Build Status | |
 | ------------ | - |
 | **master** branch CI builds | [![Azure Pipelines Build Status](https://dev.azure.com/libgit2/libgit2/_apis/build/status/libgit2?branchName=master)](https://dev.azure.com/libgit2/libgit2/_build/latest?definitionId=7&branchName=master)   |
+| **v0.28 branch** CI builds | [![Azure Pipelines Build Status](https://dev.azure.com/libgit2/libgit2/_apis/build/status/libgit2?branchName=maint/v0.28)](https://dev.azure.com/libgit2/libgit2/_build/latest?definitionId=7&branchName=maint/v0.28) |
 | **v0.27 branch** CI builds | [![Azure Pipelines Build Status](https://dev.azure.com/libgit2/libgit2/_apis/build/status/libgit2?branchName=maint/v0.27)](https://dev.azure.com/libgit2/libgit2/_build/latest?definitionId=7&branchName=maint/v0.27) |
 | **v0.26 branch** CI builds | [![Azure Pipelines Build Status](https://dev.azure.com/libgit2/libgit2/_apis/build/status/libgit2?branchName=maint/v0.26)](https://dev.azure.com/libgit2/libgit2/_build/latest?definitionId=7&branchName=maint/v0.26) |
 | **Nightly** builds | [![Azure Pipelines Build Status](https://libgit2.visualstudio.com/libgit2/_apis/build/status/libgit2?branchName=master&label=Full+Build)](https://libgit2.visualstudio.com/libgit2/_build/latest?definitionId=9&branchName=master) [![Build Status](https://dev.azure.com/libgit2/libgit2/_apis/build/status/coverity?branchName=master&label=Coverity+Build)](https://dev.azure.com/libgit2/libgit2/_build/latest?definitionId=21?branchName=master) [![Coverity Scan Build Status](https://scan.coverity.com/projects/639/badge.svg)](https://scan.coverity.com/projects/639) |