Close joaotavora/eglot#1013: Add Marksman server for Markdown

* eglot.el (eglot-server-programs): Update.
* README (Connecting to a server): Add marksman.
* NEWS.md: Mention change.

Copyright-paperwork-exempt: yes

eglot.el

@@ -197,7 +197,8 @@ language-server/bin/php-language-server.php"))
                                 (clojure-mode . ("clojure-lsp"))
                                 (csharp-mode . ("omnisharp" "-lsp"))
                                 (purescript-mode . ("purescript-language-server" "--stdio"))
-                                (perl-mode . ("perl" "-MPerl::LanguageServer" "-e" "Perl::LanguageServer::run")))
+                                (perl-mode . ("perl" "-MPerl::LanguageServer" "-e" "Perl::LanguageServer::run"))
+                                (markdown-mode . ("marksman" "server")))
   "How the command `eglot' guesses the server to start.
 An association list of (MAJOR-MODE . CONTACT) pairs.  MAJOR-MODE
 identifies the buffers that are to be managed by a specific