Clarification of build instructions (in "BUILD.md")

_Note: I used Xcode 9.2, with an iPhone 7 target running iOS 11.2.2. I also have a paid Apple developer license, which simplifies matters :smiley:_

The instructions are generally easy to follow, but there are a few places where (IMHO) some touchups would be helpful. Here is a list.

1. After building "Libssh2 for iOS", it is necessary to run the `create-libssh2-framework.sh` script in order to generate the `libssh2.framework`. Cd to the root of the project (`libssh2-for-iOS`, unless you named it something else when cloning) and run `./create-libssh2-framework.sh`.

2. After building "Mosh for iOS", the `libprotobuf.a` library will be in the `protobuf-release/lib/` subdirectory, rather than the `build-protobuf/protobuf-version/lib` subdirectory. Copy the library to the `Frameworks/lib` subdirectory of the "Blink" project, rather than the `Frameworks` subdirectory, or you will get a linker error when trying to build Blink later (this point might be obvious, of course! :smiley:)

3. The `hterm_all.js` and `webfonts.js` are already present in the `Resources` folder of the Blink project; apparently there is no need to build/download them separately.

4. Pick an iCloud container for your Blink build in the Xcode IDE, or Xcode will refuse to proceed. It is under "Targets > Blink > Capabilities > iCloud". I simply clicked the "Use default container" radio button, and the build succeeded, although I have not tested iCloud functionality yet.

This project is fantastic -- very, very useful! 👍

Open BUILD.md and compare the Libssh2, Mosh, asset, and iCloud setup steps with the paths and Xcode settings described in the issue. Update the instructions so the script invocation, library destination, existing resources, and required iCloud container are explicit; done means a newcomer can follow the build without the reported linker or Xcode errors.

Written by the indexing model from the issue text.