improve markdown

TelloVideoDecoder/README.md

@@ -9,29 +9,29 @@ Therefore I will be grateful that if you can send me pull requests to make it be
 
 ## Get a valid NAL Unit
 You need to first generate valid NALU either by yourself or by `getNalUnits()`/ `readNalUnits()`.
-
->getNalUnits(streamBuffer: Array<UInt8>) -> NALUnit?
-
+```swift
+getNalUnits(streamBuffer: Array<UInt8>) -> NALUnit?
+```
 will get all valid NALU and abandon incomplete NALU (data before next start code)
-
->readNalUnits(streamBuffer:inout Array<UInt8>) -> NALUnit?
-
+```swift
+readNalUnits(streamBuffer:inout Array<UInt8>) -> NALUnit?
+```
 will consume `streamBuffer`. To stop reading, call `stopProcessing()`
 
 ## Get CMSampleBuffer from NALU
 Get a CMSampleBuffer by `getCMSampleBuffer(from nalu: NALUnit)`
 
 ## Get CVImageBuffer from CMSampleBuffer
 Get a `CVImageBuffer` by calling `decompress()`
-
->decompress(sampleBuffer: CMSampleBuffer, outputHandler: @escaping VTDecompressionOutputHandler) -> OSStatus
-
+```swift
+decompress(sampleBuffer: CMSampleBuffer, outputHandler: @escaping VTDecompressionOutputHandler) -> OSStatus
+```
 This is a wrapper of `VTDecompressionSessionDecodeFrame(_:sampleBuffer:flags:infoFlagsOut:outputHandler:)`
 
 note that
->outputHandler cannot be called with a session created with a VTDecompressionOutputCallbackRecord.
+**outputHandler cannot be called with a session created with a VTDecompressionOutputCallbackRecord.**
 
-### what if I want to use VTDecompressionOutputCallbackRecord?
+### what if I want to use `VTDecompressionOutputCallbackRecord`?
 Grab the source code and modify `createVTDecompressionSession()` by commenting back 
 ```swift
 var outputCallback = VTDecompressionOutputCallbackRecord()
@@ -44,8 +44,9 @@ and pass to `outputCallback: ` in `VTDecompressionSessionCreate()`
 I don't quite understand what each parameter means while creating the CMSampleTimingInfo. Even if I don't set it, the video streaming seems working well. Welcome any improvement and knowledge sharing.
 
 ## About renderVideoStream()
->renderVideoStream(streamBuffer: inout Array<UInt8>, to videoLayer: AVSampleBufferDisplayLayer)
-
+```swift
+renderVideoStream(streamBuffer: inout Array<UInt8>, to videoLayer: AVSampleBufferDisplayLayer)
+```
 This is a lazy method for people just want to display the video stream from Tello with  `AVSampleBufferDisplayLayer`
 
 It simply keeps consuming `streamBuffer`, so you would want to put it into a `DispatchQueue` as it will block current thread, and be careful not causing write/read corruption. This also applies to `readNalUnits()`.