docs: add simple connection sample to readme

[olavloite]

Adds a simple code example for connecting to Cloud Spanner using JDBC.
Also fixes a wrong link to the online documentation.

[codecov]

Codecov Report

Merging #263 (f827826) into master (c9906c9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #263   +/-   ##
=========================================
  Coverage     71.40%   71.40%           
  Complexity     1104     1104           
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  Files            24       24           
  Lines          3396     3396           
  Branches        525      525           
=========================================
  Hits           2425     2425           
  Misses          756      756           
  Partials        215      215           

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[thiagotnunes]

LGTM

[olavloite]

@thiagotnunes Seems like this part of the README is also automatically generated. Do you know where we need to make this change to include it in the generation process?

[thiagotnunes]

In the java-spanner repository we have a .readme-partials.yaml which is included in the README generation (see https://github.com/googleapis/java-spanner/blob/master/.readme-partials.yaml).

I think we could add it in such a file here too.