Add in 5.3.1 Release notes

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+# CiviCRM 5.3.1
+
+Released July 18, 2018
+
+- **[Security advisories](#security)**
+- **[Features](#features)**
+- **[Bugs resolved](#bugs)**
+- **[Miscellany](#misc)**
+- **[Credits](#credits)**
+
+## <a name="security"></a>Security advisories
+
+- **[CIVI-SA-2018-01](https://civicrm.org/advisory/civi-sa-2018-01-sql-injection-in-get-cases-ajax-api)** SQL injection in get-cases AJAX API
+- **[CIVI-SA-2018-02](https://civicrm.org/advisory/civi-sa-2018-02-reflected-xss-in-contribution-reports)** Reflected XSS in Contribution Reports
+- **[CIVI-SA-2018-03](https://civicrm.org/advisory/civi-sa-2018-03-reflected-xss-in-error-message)** Reflected XSS in error message
+- **[CIVI-SA-2018-04](https://civicrm.org/advisory/civi-sa-2018-04-sql-injection-in-custom-groups)** SQL injection in Custom Groups
+- **[CIVI-SA-2018-05](https://civicrm.org/advisory/civi-sa-2018-05-reflected-xss-in-contact-merge-screen)** Reflected XSS in Contact Merge Screen
+- **[CIVI-SA-2018-06](https://civicrm.org/advisory/civi-sa-2018-06-reflected-xss-in-context-parameter)** Reflected XSS in "New Membership" Form
+