Add --help message

src/cli/convert_json_toml.cpp

@@ -1,3 +1,5 @@
+#include <cstdlib>
+#include <cstring>
 #include <openPMD/auxiliary/JSON_internal.hpp>
 
 #include <iostream>
@@ -39,6 +41,21 @@ int main(int argc, char const **argv)
         std::cin >> jsonOrToml;
         break;
     case 2:
+        if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0)
+        {
+            std::cout << "Usage: " << std::string(argv[0]) << R"( [json_or_toml]
+'json_or_toml' can be a JSON or TOML dataset specified inline or a reference
+to a file prepended by an '@'.
+Inline datasets will be interpreted as JSON if they start with an '{', as TOML
+otherwise. Datasets from a file will be interpreted as JSON or TOML depending
+on the file ending '.json' or '.toml' respectively.
+Inline dataset specifications can be replaced by input read from stdin.
+
+If the input is JSON, then it will be converted to TOML and written to stdout,
+equivalently from TOML to JSON.
+)";
+            exit(0);
+        }
         jsonOrToml = argv[1];
         break;
     default: