doc/go1.13: start doc, note macOS, FreeBSD deprecations

For #23011.
For #27619.

Change-Id: Id1f280993ecdfb07a7420926ca1c0f5b7872afbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/174521Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.13.html

+<!--{
+        "Title": "Go 1.13 Release Notes",
+        "Path":  "/doc/go1.13",
+        "Template": true
+}-->
+
+<!--
+NOTE: In this document and others in this directory, the convention is to
+set fixed-width phrases with non-fixed-width spaces, as in
+<code>hello</code> <code>world</code>.
+Do not send CLs removing the interior tags from such phrases.
+-->
+
+<style>
+  ul li { margin: 0.5em 0; }
+</style>
+
+<h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.13</h2>
+
+<p>
+  <strong>
+    Go 1.13 is not yet released. These are work-in-progress
+    release notes. Go 1.13 is expected to be released in August 2019.
+  </strong>
+</p>
+
+<p>
+TODO
+</p>
+
+<h2 id="language">Changes to the language</h2>
+
+<p>
+TODO
+</p>
+
+<h2 id="ports">Ports</h2>
+
+<p>
+TODO
+</p>
+
+<h3 id="darwin">Darwin</h3>
+
+<p>
+  As <a href="go1.12#darwin">announced</a> in the Go 1.12 release notes,
+  Go 1.13 now requires macOS 10.11 El Capitan or later;
+  support for previous versions has been discontinued.
+</p>
+
+<h3 id="freebsd">FreeBSD</h3>
+
+<p>
+  As <a href="go1.12#freebsd">announced</a> in the Go 1.12 release notes,
+  Go 1.13 now requires FreeBSD 11.2 or later;
+  support for previous versions has been discontinued.
+  FreeBSD 12.0 or later requires a kernel with the COMPAT_FREEBSD11 option set (this is the default).
+</p>
+
+<h2 id="tools">Tools</h2>
+
+<p>
+TODO
+</p>
+
+<h2 id="runtime">Runtime</h2>
+
+<p>
+TODO
+</p>
+
+
+<h2 id="library">Core library</h2>
+
+<p>
+TODO generally
+</p>
+
+<h3 id="tls_1_3">TLS 1.3</h3>
+
+<p>
+TODO; link to <a href="/doc/go1.12#tls_1_3">Go 1.12 notes</a>.
+</p>
+
+
+<h3 id="minor_library_changes">Minor changes to the library</h3>
+
+<p>
+  As always, there are various minor changes and updates to the library,
+  made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
+  in mind.
+</p>
+
+<p>
+TODO
+</p>
+