Add snapshot store implementations

Author: codeliner

.gitignore

@@ -6,3 +6,4 @@
 .php_cs.cache
 nbproject
 composer.lock
+docs/html

docs/bookdown.json

@@ -2,7 +2,10 @@
   "title": "Snapshot Store",
   "content": [
     {"snapshot_store": "snapshot_store.md"},
-    {"mongodb_snapshot_store": "https://raw.githubusercontent.com/prooph/mongodb-snapshot-store/master/docs/bookdown.json"}
+    {"pdo_snapshot_store": "https://raw.githubusercontent.com/prooph/pdo-snapshot-store/master/docs/bookdown.json"},
+    {"mongodb_snapshot_store": "https://raw.githubusercontent.com/prooph/mongodb-snapshot-store/master/docs/bookdown.json"},
+    {"memcached_snapshot_store": "https://raw.githubusercontent.com/prooph/memcached-snapshot-store/master/docs/bookdown.json"},
+    {"arangodb_snapshot_store": "https://raw.githubusercontent.com/prooph/arangodb-snapshot-store/master/docs/bookdown.json"}
   ],
   "tocDepth": 1,
   "numbering": false,