Add release checksums.

Keep a copy of the release tarball sha-256 checksums in the
repository for reference. Having a copy with the code in
distributed version control adds a verification path.

This should match http://downloads.xiph.org/releases/opus/SHA256SUMS.txt

releases.sha2

+b2f75c4ac5ab837845eb028413fae2a28754bfb0a6d76416e2af1441ef447649  opus-0.9.0.tar.gz
+4e379a98ba95bbbfe9087ef10fdd05c8ac9060b6d695f587ea82a7b43a0df4fe  opus-0.9.10.tar.gz
+b1cad6846a8f819a141009fe3f8f10c946e8eff7e9c2339cd517bb136cc59eae  opus-0.9.14.tar.gz
+206221afc47b87496588013bd4523e1e9f556336c0813f4372773fc536dd4293  opus-0.9.1.tar.gz
+6e85c1b57e1d7b7dfe2928bf92586b96b73a9067e054ede45bd8e6d24bd30582  opus-0.9.2.tar.gz
+d916e34c18a396eb7dffc47af754f441af52a290b761e20db9aedb65928c699e  opus-0.9.3.tar.gz
+53801066fa97329768e7b871fd1495740269ec46802e1c9051aa7e78c6edee5b  opus-0.9.5.tar.gz
+3bfaeb25f4b4a625a0bc994d6fc6f6776a05193f60099e0a99f7530c6b256309  opus-0.9.6.tar.gz
+1b69772c31c5cbaa43d1dfa5b1c495fc29712e8e0ff69d6f8ad46459e5c6715f  opus-0.9.7.tar.gz
+4aa30d2e0652ffb4a7a22cc8a29c4ce78267626f560a2d9213b1d2d4e618cf36  opus-0.9.8.tar.gz
+2f62359f09151fa3b242040dc9b4c5b6bda15557c5daea59c8420f1a2ff328b7  opus-0.9.9.tar.gz
+43bcea51afa531f32a6a5fdd9cba4bd496993e26a141217db3cccce6caa7cd74  opus-1.0.0-rc.tar.gz
+9250fcc74472d45c1e14745542ec9c8d09982538aefed56962495614be3e0d2d  opus-1.0.0.tar.gz
+76bc0a31502a51dae9ab737b4db043b9ecfcd0b5861f0bfda41b662bd5b92227  opus-1.0.1-rc2.tar.gz
+3de8d6809dac38971ebb305532d4ea532519d3bed08985f25d6c557f9ce5e8ff  opus-1.0.1-rc3.tar.gz
+8044397a6365a07117b08cbe8f9818bf7c93746908806ba74a2917187bbdda5f  opus-1.0.1-rc.tar.gz
+80fa5c3caf2ac0fd68f8a22cce1564fc46b368c773a17554887d0066fe1841ef  opus-1.0.1.tar.gz
+da615edbee5d019c1833071d69a4782c19f178cf9ca1401375036ecef25cd78a  opus-1.0.2.tar.gz
+191a089c92dbc403de6980463dd3604b65beb12d283c607e246c8076363cb49c  opus-1.0.3.tar.gz
+a8d40efe87f6c3e76725391457d46277878c7a816ae1642843261463133fa5c8  opus-1.1-alpha.tar.gz
+ec1784287f385aef994b64734aaecae04860e61aa50fc6eef6643fa7e40dd193  opus-1.1-beta.tar.gz
+8aa16360f59a94d3e38f38f28d24039f7663179682cbae82aa42f1dd9e52e6ed  opus-1.1-rc.tar.gz
+ebc87a086d4fe677c5e42d56888b1fd25af858e4179eae4f8656270410dffac3  opus-1.1-rc2.tar.gz
+cbfd09c58cc10a4d3fcb727ad5d46d7bb549f8185ac922ee28b4581b52a7bee9  opus-1.1-rc3.tar.gz
+b9727015a58affcf3db527322bf8c4d2fcf39f5f6b8f15dbceca20206cbe1d95  opus-1.1.tar.gz
+0c668639dcd16b14709fc9dc49e6686606f5a256f2eaa1ebaa2f39a66f8626cd  opus-1.1.1-beta.tar.gz
+8071b968475c1a17f54b6840d6de9d9ee20f930e827b0401abe3c4cf4f3bf30a  opusfile-0.1.tar.gz
+b4a678b3b6c4adfb6aff1f67ef658becfe146ea7c7ff228e99543762171557f9  opusfile-0.2.tar.gz
+94ac78ca4f74c4e43bc9fe4ec1ad0aa36f38ab90f45b0727c40dd1e96096e767  opus_testvectors-draft11.tar.gz
+94ac78ca4f74c4e43bc9fe4ec1ad0aa36f38ab90f45b0727c40dd1e96096e767  opus_testvectors.tar.gz
+5d2b99757bcb628bab2611f3ed27af6f35276ce3abc96c0ed4399d6c6463dda5  opus-tools-0.1.2.tar.gz
+008317297d6ce84f84992abf8cc948a048a4fa135e1d1caf429fafde8965a792  opus-tools-0.1.3.tar.gz
+de80485c5afa1fd83c0e16a0dd4860470c872997a7dd0a58e99b2ee8a93e5168  opus-tools-0.1.4.tar.gz
+76678d0eb7a9b3d793bd0243f9ced9ab0ecdab263f5232ed940c8f5795fb0405  opus-tools-0.1.5.tar.gz
+cc86dbc2a4d76da7e1ed9afee85448c8f798c465a5412233f178783220f3a2c1  opus-tools-0.1.6.tar.gz
+e0f08d301555dffc417604269b5a85d2bd197f259c7d6c957f370ffd33d6d9cd  opus-tools-0.1.7.tar.gz
+e4e188579ea1c4e4d5066460d4a7214a7eafe3539e9a4466fdc98af41ba4a2f6  opus-tools-0.1.8.tar.gz
+4248927f2c4e316ea5b84fb02bd100bfec8fa4624a6910d77f0af7f0c6cb8baa  opusfile-0.3.tar.gz
+9836ea11706c44f36de92c4c9b1248e03a4c521e7fb2cff18a0cb4f8b0e79140  opusfile-0.4.tar.gz
+f187906b1b35f7f0d7de6a759b4aab512a9279d23adb35d8009e7e33bd6a922a  opusfile-0.4.zip
+2ce52d006aeeec9f10260dbe3073c4636954a1ab19c82b8baafefe0180aa4a39  opusfile-0.5.tar.gz
+b940d62beb15b5974764574b9f265481fe5b6ee16902fb705727546caf956261  opusfile-0.5.zip
+2428717b356e139f18ed2fdb5ad990b5654a238907a0058200b39c46a7d03ea6  opusfile-0.6.tar.gz
+753339225193df605372944889023b9b3c5378d672e8784d69fa241cd465278c  opusfile-0.6.zip
+b1873dd78c7fbc98cf65d6e10cfddb5c2c03b3af93f922139a2104baedb4643a  opus-tools-0.1.9.tar.gz