* Updating the writeup of RNN doc

* Editing the documentation for seq_decoder, and fixing typos

* Fixing the captioning on 2 level RNN

* pushing after a pull

* Editing and re-writing parts of Data Reader design doc

* Support the build for multiple architectures at one cmake command (iOS).

* Update the documentations.

* Chage `IndicateDataType` to `GetKernelType`. Make it easier to
  understand.
* Change `OpKernelKey` to `OpKernelType`
* Make operator developers can customize which kernel the operator will
  use in runtime.

* small fix

* fix comment

* address comment

* small fix

fix nodebook to notebook

* Add English version of Android cross-compiling document

* Add English version of Android cross-compiling document

* Follow comments from Yi-qun and Kavya

* Adding design doc for model average (now called parameter_average)

* Updating title

* Updating image tag

* Updating review comments

* 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