Added codecov Badge

[Mohitbalwani26]

References to other Issues or PRs

Fixes #18263

Brief description of what is fixed or changed

Added the codecov Badge to readme.md

Other comments

Release Notes

• other
  • README.rst - Added the codecov Badge in readme.rst

[sympy-bot]

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• other
  • README.rst - Added the codecov Badge in readme.rst (#18266 by @Mohitbalwani26)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.6.

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<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
format, e.g. "Fixes #1234". See
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open. -->
Fixes #18263 


#### Brief description of what is fixed or changed
Added the codecov Badge to readme.md

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
* other
    * README.rst - Added the codecov Badge in readme.rst
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18266 into master will increase coverage by 0.014%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##            master    #18266       +/-   ##
=============================================
+ Coverage   74.981%   74.996%   +0.014%     
=============================================
  Files          649       649               
  Lines       167350    167350               
  Branches     39382     39382               
=============================================
+ Hits        125482    125507       +25     
+ Misses       36329     36305       -24     
+ Partials      5539      5538        -1