readme: add a very brief section to introduce image optimizer

[changweige]

A containerd NRI plugin has been born and works well to watch the file operations events when a container is being started.
It can generate an ordered files list that suggests nydusify and nydus-image to build a more efficient nydus image in a static and persisted method.

[anolis-bot]

@changweige , a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/54569

[anolis-bot]

@changweige , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!

[anolis-bot]

@changweige , the code has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/54682

[anolis-bot]

@changweige , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!