Update README with build instructions

README.md

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+Building
+--------
+
+First run should use zig build -Dfetch. This will pull dependencies
+
+Building for Linux
+------------------
+Linux requires libx11 and libxfixes. Specific curl commands are in build.zig
+
+
+Building for Windows
+--------------------
+
+```
+zig build -Dtarget=x86_64-windows -Dcurl -Dseperate-encryption -Drelease-safe && cp zig-out/bin/*.???
+```
+
+Building for Linux:
+```
+zig build
+```
+
+Three executables come out of this:
+
+* encrypt: Only used in Windows when -Dseperate-encryption is specified. This works around
+           crowdstrike restrictions, but only if the executable is not named "encrypt",
+           so I've rot13'd the name.
+* clipboard-download: Downloads from the clipboard
+* clipboard-upload: Uploads to the clipboard
+
+There is nothing to prevent a download from setting the clipboard and triggering
+an upload. We'll live with that, at least for now.
+
+Issues to fix
+-------------
+
+Iguana needs TLS 1.3 support, or support for a ECDSA algorithm