Fix code snippets and enhance metadata retrieval documentation

Updated code snippets in README for header retrieval and added optional parameter documentation details.

Author: mrtineide

CCDB/README.md

@@ -42,7 +42,7 @@ auto deadpixelsback = api.retrieveFromTFileAny<o2::FOO::DeadPixelMap>("FOO/DeadP
 std::map<std::string, std::string> headers;
 auto deadpixelsback = api.retrieveFromTFileAny<o2::FOO::DeadPixelMap>("FOO/DeadPixels", metadata /* constraint the objects retrieved to those matching the metadata */, -1 /* timestamp */, &headers /* the headers attached to the returned object */); 
 // finally, use this method to retrieve only the headers (and thus the metadata)
-std::map<std::string, std::string> headers = f.api.retrieveHeaders("FOO/DeadPixels", f.metadata); 
+std::map<std::string, std::string> headers = api.retrieveHeaders("FOO/DeadPixels", metadata); 
 ```
 
 * creating a local snapshot and fetching objects therefrom
@@ -111,6 +111,12 @@ This feature is useful to avoid using newer objects if the CCDB is updated in pa
 
 In cached mode, the manager can check that local objects are still valid by requiring `mgr.setLocalObjectValidityChecking(true)`, in this case a CCDB query is performed only if the cached object is no longer valid.
 
+If you want the headers/metadata for the object retrieved from the CCDB there is an optional paramater to `BasicCCDBManager::getForTimeStamp`. These headers are also cached (when caching is enabled) and is updated when a CCDB query is sent.
+```c++
+std::map<std::string,std::string> headers;
+mgr.getForTimeStamp(path, timstamp, metadata, &headers);
+```
+
 ## Future ideas / todo:
 
 - [ ] offer improved error handling / exceptions