Improve the documentation for 50Mb limit

[xgouchet]

What does this PR do?

Improve the documentation to give workarounds when the mapping file is above 50 Mb

[codecov-commenter]

Codecov Report

Merging #141 (d5602e0) into develop (b381c3f) will decrease coverage by 0.18%. The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #141      +/-   ##
=============================================
- Coverage      86.25%   86.07%   -0.18%     
+ Complexity       174      173       -1     
=============================================
  Files             21       21              
  Lines            560      560              
  Branches          72       72              
=============================================
- Hits             483      482       -1     
  Misses            49       49              
- Partials          28       29       +1     

Impacted Files	Coverage Δ
...com/datadog/gradle/plugin/DdAndroidGradlePlugin.kt	74.14% <0.00%> (-0.86%)	:arrow_down:

[plousada]

We now support 100Mb in US1, should we change this doc?

[xgouchet]

@plousada yes that would make sense, I'll update the PR today