Move more SDK docs to docs folder (#3794)

docs/trace/customizing-the-sdk/README.md

@@ -108,11 +108,94 @@ name starts with "Abc.".
 
 ### Instrumentation
 
-// TODO
+While the OpenTelemetry API can be used to instrument any library manually,
+[Instrumentation
+Libraries](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/overview.md#instrumentation-libraries)
+are available for a lot of commonly used libraries. Such instrumentations can be
+added to the `TracerProvider`. It is not required to attach the instrumentation
+to the provider, unless the life cycle of the instrumentation must be managed by
+the provider. If the instrumentation must be activated/shutdown/disposed along
+with the provider, then the instrumentation must be added to the provider.
+
+Typically, the instrumentation libraries provide extension methods on
+`TracerProviderBuilder` to allow adding them to the `TracerProvider`. Please
+refer to corresponding documentation of the instrumentation library to know the
+exact method name.
+
+Follow [this](../extending-the-sdk/README.md#instrumentation-library) document
+to learn about the instrumentation libraries shipped from this repo.
 
 ### Processor
 
-// TODO
+[Processors](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#span-processor)
+allows hooks for start and end of `Activity`. If no processors are configured,
+then traces are simply dropped by the SDK. `AddProcessor` method on
+`TracerProviderBuilder` should be used to add a processor. There can be any
+number of processors added to the provider, and they are invoked in the same
+order as they are added. Unlike `Sampler` or `Resource`, processors can be added
+to the provider even *after* it is built.
+
+The snippet below shows how to add processors to the provider before and after
+it is built.
+
+```csharp
+using OpenTelemetry;
+using OpenTelemetry.Trace;
+
+using var tracerProvider = Sdk.CreateTracerProviderBuilder()
+    .AddProcessor(new MyProcessor1())
+    .AddProcessor(new MyProcessor2()))
+    .Build();
+
+// Processors can be added to provider even after it is built.
+// Only those traces which are emitted after this line, will be sent to it.
+tracerProvider.AddProcessor(new MyProcessor3());
+```
+
+A `TracerProvider` assumes ownership of any processors added to it. This means
+that, provider will call `Shutdown` method on the processor, when it is
+shutdown, and disposes the processor when it is disposed. If multiple providers
+are being setup in an application, then separate instances of processors must be
+configured on them. Otherwise, shutting down one provider can cause the
+processor in other provider to be shut down as well, leading to undesired
+results.
+
+Processors can be used for enriching the telemetry and exporting the telemetry
+to an exporter. For enriching purposes, one must write a custom processor, and
+override the `OnStart` or `OnEnd` method with logic to enrich the telemetry. For
+exporting purposes, the SDK provides the following built-in processors:
+
+* [BatchExportProcessor<T>](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#batching-processor)
+  : This is an exporting processor which batches the telemetry before sending to
+  the configured exporter.
+
+  The following environment variables can be used to override the default
+  values of the `BatchExportActivityProcessorOptions`.
+
+  | Environment variable             | `BatchExportActivityProcessorOptions` property |
+  | -------------------------------- | ---------------------------------------------- |
+  | `OTEL_BSP_SCHEDULE_DELAY`        | `ScheduledDelayMilliseconds`                   |
+  | `OTEL_BSP_EXPORT_TIMEOUT`        | `ExporterTimeoutMilliseconds`                  |
+  | `OTEL_BSP_MAX_QUEUE_SIZE`        | `MaxQueueSize`                                 |
+  | `OTEL_BSP_MAX_EXPORT_BATCH_SIZE` | `MaxExportBatchSizeEnvVarKey`                  |
+
+  `FormatException` is thrown in case of an invalid value for any of the
+  supported environment variables.
+
+* [CompositeProcessor<T>](../../../src/OpenTelemetry/CompositeProcessor.cs)
+  : This is a processor which can be composed from multiple processors. This is
+  typically used to construct multiple processing pipelines, each ending with
+  its own exporter.
+
+* [SimpleExportProcessor<T>](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#simple-processor)
+  : This is an exporting processor which passes telemetry to the configured
+  exporter without any batching.
+
+Follow [this](../extending-the-sdk/README.md#processor) document
+to learn about how to write own processors.
+
+*The processors shipped from this SDK are generics, and supports tracing and
+logging, by supporting `Activity` and `LogRecord` respectively.*
 
 ### Resource