add fdtdx repo

[frederikschubert]

fdtdx is a FDTD engine and inverse design suite that enables the flexible optimization of 3D photonic devices.

Summary by Sourcery

Documentation:

• Add a link to the fdtdx repository in the README file.

[sourcery-ai]

Reviewer's Guide by Sourcery

This PR adds a reference to the fdtdx repository in the README.md file. The change is a simple addition to the list of FDTD (Finite-Difference Time-Domain) simulation tools.

No diagrams generated as the changes look simple and do not need a visual representation.

File-Level Changes

Change	Details	Files
Added a new FDTD simulation tool reference to the documentation	Added fdtdx GitHub repository link to the list of FDTD simulators	README.md

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[sourcery-ai]

Hey @frederikschubert - I've reviewed your changes - here's some feedback:

Overall Comments:

• Consider adding a brief description of fdtdx's key features (like inverse design capabilities) to match the style of other entries in the list

Here's what I looked at during the review

• 🟡 General issues: 2 issues found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[sourcery-ai]

issue (typo): There's a typo in 'propietary' - it should be 'proprietary'

[sourcery-ai]

suggestion: Consider adding brief distinguishing characteristics for fdtdx

Other entries in this list include brief descriptions of key features or implementation details. Adding similar information for fdtdx would maintain consistency and help users quickly compare different tools.