Update README.md

Author: diogorac

README.md

@@ -9,4 +9,8 @@ cmake ..
 make
 ctest
 ```
-
+## Note
+When building the metadata and asset objects take the following measures:
+1. No spaces in the json structure: '{"key 1": "value a"}' becomes '{"key 2":"value a"}
+2. Keys should be alphabetically ordered: '{"key 1":"value a","A key":"A value"}' becomes {"A key":"A value","key 1":"value a"}'
+3. No numbers fields, only strings '{"key 1": 2.3123}' becomes '{"key 1":"2.3123"}'