Add note about async initialisation of tokens in K8S

[DestyNova]

When used in Kubernetes, I observed a persistent failure with an error message like this:

org.zalando.stups.tokens.AccessTokenUnavailableException: No token available for
tokenId 'someToken'. Tokens are available for the following tokenIds [someToken]

This has the look of a race condition about it. After I raised the issue on an internal Zalando chat channel, Alexey Ermakov provided a helpful explanation:

unfortunately the tokens library initialises asynchronously by default on Kubernetes (but synchronously on STUPS, even though it's much more expensive), so the first couple of calls will fail.

The PR adds an example to the README with a single line of code shared by Alexey which forces the filesystem refresher to initialise and validate the tokens immediately on startup, which avoids this issue.

[codecov-commenter]

Codecov Report

Merging #203 into master will increase coverage by 0.23%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #203      +/-   ##
============================================
+ Coverage     72.55%   72.79%   +0.23%     
  Complexity      245      245              
============================================
  Files            48       48              
  Lines           849      849              
  Branches         40       40              
============================================
+ Hits            616      618       +2     
+ Misses          209      207       -2     
  Partials         24       24              

Impacted Files	Coverage Δ	Complexity Δ
.../org/zalando/stups/tokens/fs/FilesystemReader.java	100.00% <0.00%> (+10.52%)	4.00% <0.00%> (ø%)

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[DestyNova]

Not sure how adding a little to the README could increase code coverage, but I'm not gonna argue 😆

[fatroom]

👍

[vadeg]

👍

[DestyNova]

@szuecs can you merge this, or is it no longer needed?

[szuecs]

👍

[codecov-io]

Codecov Report

Merging #203 (0233d64) into master (f362498) will increase coverage by 0.23%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #203      +/-   ##
============================================
+ Coverage     72.55%   72.79%   +0.23%     
  Complexity      245      245              
============================================
  Files            48       48              
  Lines           849      849              
  Branches         40       40              
============================================
+ Hits            616      618       +2     
+ Misses          209      207       -2     
  Partials         24       24              

Impacted Files	Coverage Δ	Complexity Δ
.../org/zalando/stups/tokens/fs/FilesystemReader.java	100.00% <0.00%> (+10.52%)	4.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[tkrop]

👍

[szuecs]

@vadeg should be able to merge