Add documentation about java agent structure and related classloaders (#380)

CONTRIBUTING.md

@@ -8,6 +8,53 @@ In order to fully build and test this whole repository you need the following:
 * Java 8 should be set as default: `java -version` should give you version 8.
 * Defined environment variables `JAVA_8_HOME` and `JAVA_9_HOME` which point to the corresponding java homes. 
 
+### Plugin structure
+OpenTelemetry Auto Instrumentation java agent's jar can logically be divided into 3 parts.
+
+#### `java-agent` module
+This module consists of single class `io.opentelemetry.auto.bootstrap.AgentBootstrap` 
+which implements [Java instrumentation agent](https://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html).
+This class is loaded during application startup by application classloader. 
+Its sole responsibility is to push agent's classes into JVM's bootstrap classloader 
+and immediately delegate to `io.opentelemetry.auto.bootstrap.Agent` (now in the bootstrap class loader) class from there.
+
+#### `agent-bootstrap` module
+This module contains support classes for actual instrumentations to be loaded later and separately. 
+These classes should be available from all possible classloaders in the running application. 
+For this reason `java-agent` puts all these classes into JVM's bootstrap classloader.
+For the same reason this module should be as small as possible and have as few dependencies as possible. 
+Otherwise, there is a risk of accidentally exposing this classes to the actual application.
+
+#### `agent-tooling` module and `instrumentation` submodules
+Contains everything necessary to make instrumentation machinery work, 
+including integration with [ByteBuddy](https://bytebuddy.net/) and actual library-specific instrumentations. 
+As these classes depend on many classes from different libraries, 
+it is paramount to hide all these classes from the host application. 
+This is achieved in the following way:
+* When `java-agent` module builds the final agent, it moves all classes from `instrumentation` submodules 
+and `agent-tooling` module into a separate folder inside final jar file, called `auto-tooling-and-instrumentation.isolated`. 
+In addition, the extension of all class files is changed from `class` to `classdata`. 
+This ensures that general classloaders cannot find nor load these classes.
+* When `io.opentelemetry.auto.bootstrap.Agent` starts up, 
+it creates an instance of `io.opentelemetry.auto.bootstrap.AgentClassLoader`, 
+loads an `io.opentelemetry.auto.tooling.AgentInstaller` from that `AgentClassLoader` 
+and then passes control on to the `AgentInstaller` (now in the `AgentClassLoader`). The `AgentInstaller` then installs all of the instrumentations with the help of ByteBuddy.
+
+The complicated process above ensures that the majority of auto-instrumentation agent's classes
+are totally isolated from application classes, 
+and an instrumented class from arbitrary classloader in JVM can still access helper classes from bootstrap classloader.
+
+#### Agent jar structure
+If you now look inside `java-agent/build/libs/opentelemetry-auto-<version>.jar`, 
+you will see the following "clusters" of classes:
+* `auto-tooling-and-instrumentation.isolated/` - contains `agent-tooling` module and 
+`instrumentation` submodules, loaded and isolated inside `AgentClassLoader`. 
+Including OpenTelemetry SDK.
+* `io/opentelemetry/auto/bootstrap/` - contains `agent-bootstrap` module and available in
+bootstrap classloader.
+* `io/opentelemetry/auto/shaded/` - contains OpenTelemetry API and its dependencies. 
+Shaded during creation of `java-agent` jar file by Shadow Gradle plugin.
+
 ### Testing
 #### Java versions
 Open Telemetry Auto Instrumentation's minimal supported version is java 7.

agent-bootstrap/src/main/java/io/opentelemetry/auto/bootstrap/Agent.java

@@ -197,12 +197,11 @@ public class Agent {
   }
 
   /**
-   * Create the agent classloader. This must be called after the bootstrap jar has been appened to
+   * Create the agent classloader. This must be called after the bootstrap jar has been appended to
    * the bootstrap classpath.
    *
    * @param innerJarFilename Filename of internal jar to use for the classpath of the agent
    *     classloader
-   * @param bootstrapURL
    * @return Agent Classloader
    */
   private static ClassLoader createAgentClassLoader(