Update README.rst

Hi!
I'm the maintainer of `pyls-memestra` and I'd like to add it here as a 3rd party plugin :)
Please let me know if I should add this info somewhere else.
Thank you for open sourcing the project and for all the work!
Cheers.

Author: marimeireles

README.rst

@@ -52,6 +52,7 @@ Installing these plugins will add extra functionality to the language server:
 * pyls-mypy_ Mypy type checking for Python 3
 * pyls-isort_ Isort import sort code formatting
 * pyls-black_ for code formatting using Black_
+* pyls-memestra_ a static analysis tool for Python, which detects the use of deprecated APIs.
 
 Please see the above repositories for examples on how to write plugins for the Python Language Server. Please file an
 issue if you require assistance writing a plugin.
@@ -165,4 +166,5 @@ This project is made available under the MIT License.
 .. _pyls-mypy: https://github.com/tomv564/pyls-mypy
 .. _pyls-isort: https://github.com/paradoxxxzero/pyls-isort
 .. _pyls-black: https://github.com/rupert/pyls-black
+.. _pyls-memestra: https://github.com/QuantStack/pyls-memestra
 .. _Black: https://github.com/ambv/black