add readme and info about commit of the source repository

sentry-async-profiler/src/main/java/io/sentry/asyncprofiler/vendor/asyncprofiler/README.md

@@ -0,0 +1,4 @@
+# Vendored AsyncProfiler code for converting JFR Files
+- Vendored-in from commit fe1bc66d4b6181413847f6bbe5c0db805f3e9194 of repository: [email protected]:async-profiler/async-profiler.git
+- Only the code related to JFR conversion is included.
+- The `AsyncProfiler` itself is included as a dependency in the Maven project.

sentry-async-profiler/src/main/java/io/sentry/asyncprofiler/vendor/asyncprofiler/jfr/JfrReader.java

@@ -60,6 +60,8 @@ public final class JfrReader implements Closeable {
   public final Dictionary<JfrClass> types = new Dictionary<>();
   public final Map<String, JfrClass> typesByName = new HashMap<>();
   public final Dictionary<String> threads = new Dictionary<>();
+  // Maps thread IDs to Java thread IDs
+  // Change compared to original async-profiler JFR reader
   public final Dictionary<Long> javaThreads = new Dictionary<>();
   public final Dictionary<ClassRef> classes = new Dictionary<>();
   public final Dictionary<String> strings = new Dictionary<>();