Clarify engine instructions

[matthew-carroll]

I know everything in here is ongoing and changing, but FYI the following directions make it very easy to do a couple incorrect things.

Current directions:

Flutter engine uses gclient to manage dependencies.

If you've already cloned the flutter repository:

 1. Copy one of the engine/scripts/*.gclient to the root folder
 2. run gclient sync
 
If you haven't cloned the repository; you could copy the sample gclient file to an empty directory and use gclient sync.

Suggested directions:

Flutter engine uses gclient to manage dependencies.

If you've already cloned the flutter repository:

 1. Create or choose a new directory on your system to hold the engine code (called the "root folder").
 2. Copy one of the engine/scripts/*.gclient to the root folder
 3. Rename the copied glient script to `.gclient`.
 4. Run gclient sync.

If you haven't cloned the repository; you could copy the sample gclient file to an empty directory and use gclient sync.

[matthew-carroll]

Maybe our issue was coincidental and the original instructions really meant what they said.

We tried running gclient sync from within our flaux directory and had some kind of error. We tried in a different directory and it worked. But if the intention is to run gclient sync in-place within flaux, then the original instructions make sense for that line.

[jtmcdole]

Flaux is in flux right now as we try to get CI up and running (the hardest part of merging). I think my instructions above are from the older experiment; it should be:

• clone the repo
• copy engine/scripts/standard.gclient to the root of flaux
• then run gclient sync

With the old engine repository, you have to gclient fetch flutter which will checkout buildroot and fetch the engine.