Reword Warning on README.md, encourage grunt-assemble over assemble 0.4.42

[travellingprog]

The README warning makes it sound like using grunt-assemble is equivalent to running assemble 0.4.42.

However, the reality is that anyone running assemble v0.4.42 could benefit from running the current version of grunt-assemble instead. For example, this module uses Handlebars v3 rather than Handlebars v1 (used by assemble v0.4.42).

[assemblebot]

@travellingprog Thanks for the issue! If you're reporting a bug, please be sure to include:

• The version of assemble you are using.
• Your assemblefile.js (This can be in a gist)
• The commandline output. (Screenshot or gist is fine)
• What you expected to happen instead.