Add subscription modes for java client doc.

[codelipenghui]

Motivation

Add document for java client to describe how to use each subscription modes and the difference between each subscription modes.

Modifications

Add subscription modes section for java client doc.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[codelipenghui]

@jennifer88huang @Anonymitaet Please help review this PR, i copied the document to google doc: https://docs.google.com/document/d/1a0OD7azjAZpCmiYw9fV8kagtXSwTkD004oC3Mz636eU/edit?usp=sharing

[Jennifer88huang-zz]

Good idea, I'll give comments in the google doc.

[Jennifer88huang-zz]

@codelipenghui I've finished reviewing the file in Google doc, you can check my comments there. If there is any issue, feel free to contact me.

[codelipenghui]

@jennifer88huang Thanks for the review, i have synced updates to this PR.

[codelipenghui]

@merlimat @sijie @jiazhai Please help review the document if you have time.

[codelipenghui]

run java8 tests

[sijie]

hmm I think the description here is misleading.

I think it should contain two points:

• each partition only has one active consumer
• messages of one partition only distributed to one consumer
• multiple partitions are distributed to multiple consumers

@codelipenghui right?

[codelipenghui]

yes, updated

[codelipenghui]

run java8 tests