Add info about the official WP branches to be tested

They follow the WORKPLACE_XX_LATEST schema

src/org/moodle/ci/versions/WP37.groovy

+package org.moodle.ci.versions;
+
+class WP37 extends Version {
+    WP37() {
+        this.name = "37"
+        this.defaultBranch = "WORKPLACE_37_LATEST"
+        this.supportedPHPVersions = [
+            "7.1",
+            "7.2",
+            "7.3",
+        ]
+        this.supportedDatabases = [
+            "pgsql",
+            "mysqli",
+            "mariadb",
+            "sqlsrv",
+            "oci",
+        ]
+    }
+}

src/org/moodle/ci/versions/WP38.groovy

+package org.moodle.ci.versions;
+
+class WP38 extends Version {
+    WP38() {
+        this.name = "38"
+        this.defaultBranch = "WORKPLACE_38_LATEST"
+        this.supportedPHPVersions = [
+            "7.1",
+            "7.2",
+            "7.3",
+        ]
+        this.supportedDatabases = [
+            "pgsql",
+            "mysqli",
+            "mariadb",
+            "sqlsrv",
+            "oci",
+        ]
+    }
+}