Bump to 5.2.0 and write changelog

[skip ci]

Author: aantron

CHANGES

@@ -1,3 +1,10 @@
+===== 5.2.0 (2020-03-09) =====
+
+  * Add Lwt_unix.pread, Lwt_unix.pwrite, and Lwt_unix.pwrite_string (#768,
+    Pierre Chambart).
+  * Internally use accept4(2) when available (#769, Pierre Chambart).
+  * PPX: internally use 4.10 ASTs (#765).
+
 ===== 5.1.2 (2020-02-23) =====
 
 ====== Bugs fixed ======

README.md

@@ -1,6 +1,6 @@
-# Lwt    [![version 5.1.2][version]][releases] [![Travis status][travis-img]][travis] [![AppVeyor status][appveyor-img]][appveyor]
+# Lwt    [![version 5.2.0][version]][releases] [![Travis status][travis-img]][travis] [![AppVeyor status][appveyor-img]][appveyor]
 
-[version]:      https://img.shields.io/badge/version-5.1.2-blue.svg
+[version]:      https://img.shields.io/badge/version-5.2.0-blue.svg
 [releases]:     https://github.com/ocsigen/lwt/releases
 [travis]:       https://travis-ci.org/ocsigen/lwt/branches
 [travis-img]:   https://img.shields.io/travis/ocsigen/lwt/master.svg?label=travis