[GOBBLIN-1612] Add description about downloading gradle wrapper

[sekikn]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• [x] My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-1612

Description

• [x] Here are some details about my PR, including screenshots (if applicable):

This PR restores the description about downloading gradle wrapper which was removed #3159 so that users can build Gobblin from the source distribution without getting flustered.

Tests

• [x] My PR adds the following unit tests OR does not need testing for this extremely good reason:

No additional tests, since it's just a documentation fix.

Commits

• [x] My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[codecov-commenter]

Codecov Report

Merging #3471 (8c7e34c) into master (655826a) will increase coverage by 1.86%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3471      +/-   ##
============================================
+ Coverage     46.62%   48.48%   +1.86%     
+ Complexity    10352     7712    -2640     
============================================
  Files          2076     1442     -634     
  Lines         81032    56915   -24117     
  Branches       9044     6559    -2485     
============================================
- Hits          37778    27594   -10184     
+ Misses        39774    26754   -13020     
+ Partials       3480     2567     -913     

Impacted Files	Coverage Δ
...a/management/copy/publisher/CopyDataPublisher.java	74.17% <0.00%> (-1.33%)	:arrow_down:
...anagement/copy/replication/ConfigBasedDataset.java	68.87% <0.00%> (ø)
...ava/org/apache/gobblin/broker/BrokerConstants.java
...odules/orchestration/ServiceAzkabanConfigKeys.java
...n/converter/AvroStringFieldDecryptorConverter.java
...ce/extractor/extract/google/GoogleDriveSource.java
...in/azkaban/AzkabanGobblinLocalYarnAppLauncher.java
...n/data/management/retention/DatasetCleanerJob.java
.../main/java/org/apache/gobblin/utils/HttpUtils.java
...rce/extractor/extract/kafka/KafkaSimpleSource.java
... and 629 more

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