[BuildStream] BuildStream 1.2 feature page

[Agustín Benito Bethencourt <agustin benito codethink co uk>]

Hi,

I am trying to finish the feature page. I need help.

## Feature table

I am trying to finish the following table:


|      Feature             |  v1.0   |    v1.2   |               Notes               |
|:------------------------:|:-------:|:---------:|:---------------------------------:|
| CAS based artifact cache |         |   New     |    Switched to CAS based artifact cache from xxxxxxx       
           |
| [Source Mirroring]       |         |   New     |   Added functionality and configuration for providing 
multiple mirrors for any source. |
|  [Include directive]     |         |   New     |  Elements can now include content from other files.        
        |
| Junction support for subprojects   |   |   |                                   |
|  Workspace incremental builds  |   |           |                                   |
|  Element and source configuration overrides   |   |   |                           |
| Sanbox configuration     |         |           |                                   |
|  Reference storage       |         |           |                                   |

I would like to know:
* The last 5 features are new or were present in the previous release.
* If there were present, what has improved? I would like to add it to the version column.
* Can you provide me a sentence for the Notes column describing what is the feature for

I would like to add a sentence that reflects why this feature is important (impact). Can you provide me such 
sentence or two per feature?

## Comparison with 1.0

I extracted from the news file of 1.0:

"BuildStream 1.0.0 is all about API stability - for the past months we
have been reviewing our various API surfaces, implementing strategies
for revisioning of our interfaces and cleaning up. Long term stability
is very important for build reproducibility over time, and this release
is the first promise we are making on any API surfaces.

Stable API surfaces include:
  o The command line interface
  o The YAML user configuration file format
  o The YAML project `.bst` file format
  o The core Python module imported by external plugins"

Is there any change or addition on this front in 2.0? If there is, I would like to somehow add it to the 
feature page to get a sense of continuity. Can somebody provide me information?

Best regards

-- 
Agustín Benito Bethencourt
Principal Consultant
Codethink Ltd
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