Added more documentation

[martijnmelchers]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Added methods to the Client.php I don't know if I got them all or if this is the right way. Open for suggestions!

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #98 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #98   +/-   ##
======================================
  Coverage    84.7%   84.7%           
======================================
  Files           3       3           
  Lines         170     170           
======================================
  Hits          144     144           
  Misses         26      26

Impacted Files	Coverage Δ
lib/Client.php	81.56% <ø> (ø)	⬆️

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[thinkingserious]

Hello @martijnmelchers,

Thanks again for the PR!

We want to show our appreciation by sending you some swag. Could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX