Update README.md : add a FAQ about memory release

README.md

@@ -1381,6 +1381,15 @@ This library will not support comments in the future. If you wish to use comment
 
 By default, the library does not preserve the **insertion order of object elements**. This is standards-compliant, as the [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". If you do want to preserve the insertion order, you can specialize the object type with containers like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
 
+### Memory Release
+
+We checked with Valgrind and the Address Sanitizer (ASAN) that there are no memory leaks. 
+
+If you find that a parsing program with this library does not release memory, please consider the following case and it maybe unrelated to this library. 
+
+**Your program is compiled with glibc.** There is a tunable threshold that glibc uses to decide whether to actually return memory to the system or whether to cache it for later reuse. If in your program you make lots of small allocations and those small allocations are not a contiguous block and are presumably below the threshold, then they will not get returned to the OS.
+Here is a related issue [#1924](https://github.com/nlohmann/json/issues/1924).
+
 ### Further notes
 
 - The code contains numerous debug **assertions** which can be switched off by defining the preprocessor macro `NDEBUG`, see the [documentation of `assert`](https://en.cppreference.com/w/cpp/error/assert). In particular, note [`operator[]`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a233b02b0839ef798942dd46157cc0fe6.html#a233b02b0839ef798942dd46157cc0fe6) implements **unchecked access** for const objects: If the given key is not present, the behavior is undefined (think of a dereferenced null pointer) and yields an [assertion failure](https://github.com/nlohmann/json/issues/289) if assertions are switched on. If you are not sure whether an element in an object exists, use checked access with the [`at()` function](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a73ae333487310e3302135189ce8ff5d8.html#a73ae333487310e3302135189ce8ff5d8).