* Add documentation of cross-compiling for iOS.

* Correst the typo in documentation of cross-compiling for raspberry pi.

* Set ANDROID_API to 21 when it is specified < 21 for arm64-v8a in build_android.sh.

* Check the input and print the usage in MergeModel.cpp.

* Add cross_compiling_for_raspberry.md

* Update cross_compiling for raspberry pi document

* Some minor edits

* In response to comments from Kavya

* Add the _en suffix

* Add symbolic link from Paddle/CONTRIBUTING.md to doc/howto/dev/contribute_to_paddle_en.md so sphinx can generate the document
* Update CONTRIBUTING.md links so sphinx does not add these links to the TOC
* Removed dev/contribute_to_paddle_cn.md from documentation, since this document is not in sync with Paddle/CONTRIBUTING.md

* Create vlog_guide.md

* Move design/vlog_guide.md into CONTRIBUTE.md

* In response to comments from Yu Yang and Tony

* In response to comments from Luo Tao

* update cluster train v2 doc

* WIP cluster train doc

* update

* cluster train doc

* add TOC for en doc

* fix sphix build issue

* fix error links

* fix link errors

* fix image link

* polish cluster train docs

* update general distributed training document

* fix sphinx compile error

* fix doc image error

Fixes #4445