Add README for soroban-rpc #175

Job	Run time
Build (ubuntu-latest, x86_64-unknown-linux-gnu, amd64)	2m 32s
Unit tests (ubuntu-20.04, 1.22)	2m 10s
Build (ubuntu-latest, aarch64-unknown-linux-gnu, arm64)	2m 55s
Integration tests (ubuntu-20.04, 1.22, .CLI.)	1m 57s
Unit tests (ubuntu-22.04, 1.22)	3m 3s
Build (macos-latest, x86_64-apple-darwin, amd64)	5m 33s
Integration tests (ubuntu-20.04, 1.22, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	12m 2s
Build (macos-latest, aarch64-apple-darwin, arm64)	6m 8s
Integration tests (ubuntu-22.04, 1.22, .CLI.)	2m 26s
Integration tests (ubuntu-22.04, 1.22, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	11m 14s
Build (windows-latest, x86_64-pc-windows-gnu, amd64)	12m 32s
	1h 2m 32s

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