[Easy] Update readme with new release & version compatibility info

[sueann]

• Moved up Release section in the README to make it more prominent what features have been added recently
• Updated documentation & travis to use Spark 2.2.1 instead of 2.1.1
• Link to the Databricks docs for the Databricks notebook - the one previously referenced is from the original DL Pipelines Databricks blog post and can go out of date.

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #100      +/-   ##
=========================================
+ Coverage   82.58%   82.6%   +0.01%     
=========================================
  Files          34      34              
  Lines        1947    1949       +2     
  Branches       40      37       -3     
=========================================
+ Hits         1608    1610       +2     
  Misses        339     339

Impacted Files	Coverage Δ
.../org/apache/spark/sql/sparkdl_stubs/UDFUtils.scala	56.52% <0%> (+4.14%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update fd18520...685d344. Read the comment docs.