New readme for v2.0 (#855)

* New readme first draft

* Some side warning fixes and cleanup

* Fixed issue 294 resurfacing

* Improving new readme's content and structure

* Fixing minor code style issues

* Added draft for v2 migration guide

* Fixing typos in the readme

* Exchanged the new readme with the old one, improved docs and upgrade notes

* Rebased and added tiny fix to readme

Author: michelebastione

MiniExcel.slnx

@@ -15,9 +15,11 @@
     <File Path=".github\workflows\benchmark.yml" />
     <File Path=".github\workflows\codeql-analysis.yml" />
     <File Path="src\Directory.Build.props" />
+    <File Path="README-V2.md" />
     <File Path="src\Directory.Packages.props" />
     <File Path="miniexcel.publickey" />
     <File Path="src\icon.png" />
+    <File Path="V2-Upgrade-Notes.md" />
   </Folder>
   <Folder Name="/Releases/">
     <File Path="docs\README.md" />