updates the roadmap & reorganizes the README (#712)

ROADMAP.md

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+# cellxgene roadmap
+
+We are very exited for _cellxgene_ to become a valuable tool in collaborations
+between computational biologists and experimental biologists working on
+single-cell transcriptomics data. _cellxgene_ is in active development, and we
+would love to include the community as we plan new features to work on. If you
+have questions of feedback about this roadmap, please submit an issue on
+GitHub.
+
+Please note: this roadmap is subject to change.
+
+*Last updated: April 11, 2019*
+
+## what we are building now
+
+In the near term, our goal is to enable teams of computational and experimental
+biologists to collaboratively explore and annotate their single-cell RNA-seq data.
+
+There are 4 key features we plan to implement in the near term.
+
+- Click install and launch
+- Manual annotation workflows
+- Toggle embeddings
+- Gene information
+
+### simple install and launch
+
+The command line interface for installing and launching cellxgene is a barrier
+for users who are not used to Python or using the command line. We plan to
+support installation and launch of cellxgene on Mac and Windows. See
+[Issue #687](https://github.com/chanzuckerberg/cellxgene/issues/687) for more details.
+
+### manual annotation workflows
+
+The exploratory visualization that cellxgene offers is critical for manual
+annotation workflows, especially in collaborative environments. We plan to
+support manually annotate cells with labels (i.e., cell type or QC flags) for
+downstream analysis. See [Issue #524](https://github.com/chanzuckerberg/cellxgene/issues/524)
+for more details.
+
+### toggle embeddings
+
+While a single dataset may have multiple embeddings calculated (tSNE, umap, in
+situ coordinates, trajectories, etc), cellxgene currently requires the user to select the
+embedding to use in the main layout at launch. We plan to support letting users
+toggle between any embedding present in a file from the cellxgene interface.
+See [Issue #594](https://github.com/chanzuckerberg/cellxgene/issues/594) for details.
+
+### gene information
+
+Differential expression returns only the names of genes, but no additional information
+about gene metadata, function, or known associations. We plan to help users learn
+more about genes they discover by exposing additional gene metadata. See
+[Issue #96](https://github.com/chanzuckerberg/cellxgene/issues/96) for details.