Update CI to Pharo12. Update README

Author: Hernán Morales Durand

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        smalltalk: [ Pharo64-9.0, Pharo64-10, Pharo64-11 ]
+        smalltalk: [ Pharo64-9.0, Pharo64-10, Pharo64-11, Pharo64-12 ]
     name: ${{ matrix.smalltalk }}
     steps:
       - uses: actions/checkout@v2

README.md

@@ -2,11 +2,17 @@
 
 [![Build status](https://github.com/pharo-ai/hierarchical-clustering/workflows/CI/badge.svg)](https://github.com/pharo-ai/hierarchical-clustering/actions/workflows/test.yml)
 [![Coverage Status](https://coveralls.io/repos/github/pharo-ai/hierarchical-clustering/badge.svg?branch=master)](https://coveralls.io/github/pharo-ai/hierarchical-clustering?branch=master)
+[![Pharo version](https://img.shields.io/badge/Pharo-9-%23aac9ff.svg)](https://pharo.org/download)
+[![Pharo version](https://img.shields.io/badge/Pharo-10-%23aac9ff.svg)](https://pharo.org/download)
+[![Pharo version](https://img.shields.io/badge/Pharo-11-%23aac9ff.svg)](https://pharo.org/download)
+[![Pharo version](https://img.shields.io/badge/Pharo-12-%23aac9ff.svg)](https://pharo.org/download)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/pharo-ai/hierarchical-clustering/master/LICENSE)
 
+# Description
+
 Hierarchical Clustering algorithms for Pharo
 
-## Installation
+# Installation
 
 ```smalltalk
 Metacello new
@@ -24,7 +30,7 @@ Metacello new
 	load: #('AIVizRoassalHC')
 ```
 
-## How to depend on it?
+# How to depend on it?
 
 If you want to add a dependency on hierarchical-clustering to your project, include the following lines into your baseline method: