auto merge of #16680 : brson/rust/readme, r=steveklabnik
auto merge of #16680 : brson/rust/readme, r=steveklabnik

This adds links to SO, reddit, and discuss to the README. The main intent is to start advertising discuss.rust-lang.org more, in a location that doesn't mislead casual users to it (people who are building Rust are more likely to be the right audience than those that are just visiting the web site).

rohitjoshi/rust,gifnksm/rust,hauleth/rust,krzysz00/rust,avdi/rust,KokaKiwi/rust,barosl/rust,richo/rust,TheNeikos/rust,andars/rust,mdinger/rust,kimroen/rust,gifnksm/rust,philyoon/rust,miniupnp/rust,krzysz00/rust,ejjeong/rust,rohitjoshi/rust,pshc/rust,zaeleus/rust,avdi/rust,mahkoh/rust,emk/rust,mahkoh/rust,l0kod/rust,zaeleus/rust,P1start/rust,robertg/rust,ktossell/rust,ejjeong/rust,carols10cents/rust,victorvde/rust,reem/rust,untitaker/rust,XMPPwocky/rust,l0kod/rust,graydon/rust,seanrivera/rust,barosl/rust,avdi/rust,vhbit/rust,nwin/rust,defuz/rust,mahkoh/rust,dwillmer/rust,defuz/rust,barosl/rust,vhbit/rust,gifnksm/rust,krzysz00/rust,zachwick/rust,andars/rust,mdinger/rust,0x73/rust,nwin/rust,zubron/rust,jashank/rust,richo/rust,ruud-v-a/rust,jroesch/rust,dinfuehr/rust,l0kod/rust,victorvde/rust,barosl/rust,ebfull/rust,zubron/rust,ruud-v-a/rust,mdinger/rust,pelmers/rust,untitaker/rust,quornian/rust,gifnksm/rust,mahkoh/rust,pelmers/rust,ejjeong/rust,victorvde/rust,quornian/rust,dwillmer/rust,reem/rust,avdi/rust,TheNeikos/rust,KokaKiwi/rust,omasanori/rust,dinfuehr/rust,quornian/rust,mihneadb/rust,jroesch/rust,aepsil0n/rust,mvdnes/rust,reem/rust,untitaker/rust,andars/rust,dwillmer/rust,cllns/rust,gifnksm/rust,nwin/rust,AerialX/rust,pshc/rust,untitaker/rust,ebfull/rust,miniupnp/rust,mvdnes/rust,aepsil0n/rust,l0kod/rust,GBGamer/rust,emk/rust,barosl/rust,reem/rust,miniupnp/rust,hauleth/rust,AerialX/rust-rt-minimal,richo/rust,KokaKiwi/rust,zubron/rust,P1start/rust,l0kod/rust,GBGamer/rust,untitaker/rust,cllns/rust,nwin/rust,robertg/rust,philyoon/rust,miniupnp/rust,zachwick/rust,P1start/rust,AerialX/rust-rt-minimal,seanrivera/rust,AerialX/rust,kimroen/rust,dinfuehr/rust,mihneadb/rust,GBGamer/rust,zubron/rust,bombless/rust,barosl/rust,jroesch/rust,rohitjoshi/rust,miniupnp/rust,zachwick/rust,bombless/rust,untitaker/rust,quornian/rust,AerialX/rust-rt-minimal,AerialX/rust,aepsil0n/rust,aidancully/rust,andars/rust,andars/rust,pshc/rust,XMPPwocky/rust,sae-bom/rust,kwantam/rust,kimroen/rust,victorvde/rust,0x73/rust,AerialX/rust-rt-minimal,aidancully/rust,vhbit/rust,omasanori/rust,bombless/rust-docs-chinese,pshc/rust,emk/rust,dwillmer/rust,cllns/rust,richo/rust,zaeleus/rust,robertg/rust,ktossell/rust,vhbit/rust,cllns/rust,defuz/rust,bombless/rust,AerialX/rust,zaeleus/rust,vhbit/rust,emk/rust,philyoon/rust,seanrivera/rust,cllns/rust,P1start/rust,dwillmer/rust,sae-bom/rust,rprichard/rust,kimroen/rust,ktossell/rust,ebfull/rust,hauleth/rust,l0kod/rust,nwin/rust,kimroen/rust,andars/rust,zachwick/rust,richo/rust,victorvde/rust,jashank/rust,robertg/rust,defuz/rust,bombless/rust,mdinger/rust,aneeshusa/rust,omasanori/rust,dinfuehr/rust,miniupnp/rust,ruud-v-a/rust,krzysz00/rust,pshc/rust,rprichard/rust,dinfuehr/rust,sae-bom/rust,kimroen/rust,ebfull/rust,vhbit/rust,mahkoh/rust,zaeleus/rust,l0kod/rust,emk/rust,pshc/rust,ktossell/rust,omasanori/rust,hauleth/rust,defuz/rust,rohitjoshi/rust,victorvde/rust,jroesch/rust,aneeshusa/rust,carols10cents/rust,bombless/rust,jroesch/rust,zubron/rust,jashank/rust,quornian/rust,vhbit/rust,TheNeikos/rust,mihneadb/rust,kwantam/rust,miniupnp/rust,pelmers/rust,pelmers/rust,0x73/rust,ruud-v-a/rust,aneeshusa/rust,rprichard/rust,zubron/rust,aidancully/rust,pelmers/rust,jashank/rust,miniupnp/rust,nwin/rust,hauleth/rust,richo/rust,reem/rust,robertg/rust,ebfull/rust,mdinger/rust,XMPPwocky/rust,ktossell/rust,ktossell/rust,barosl/rust,KokaKiwi/rust,KokaKiwi/rust,GBGamer/rust,jashank/rust,jroesch/rust,cllns/rust,mvdnes/rust,P1start/rust,mvdnes/rust,mihneadb/rust,rprichard/rust,pshc/rust,XMPPwocky/rust,GBGamer/rust,dwillmer/rust,TheNeikos/rust,robertg/rust,sae-bom/rust,ktossell/rust,philyoon/rust,graydon/rust,ejjeong/rust,carols10cents/rust,sae-bom/rust,zachwick/rust,AerialX/rust,aneeshusa/rust,omasanori/rust,0x73/rust,jashank/rust,rprichard/rust,TheNeikos/rust,rohitjoshi/rust,pshc/rust,avdi/rust,mvdnes/rust,mdinger/rust,mihneadb/rust,emk/rust,KokaKiwi/rust,XMPPwocky/rust,seanrivera/rust,ruud-v-a/rust,jroesch/rust,carols10cents/rust,P1start/rust,nwin/rust,kimroen/rust,aepsil0n/rust,aidancully/rust,l0kod/rust,seanrivera/rust,gifnksm/rust,carols10cents/rust,pelmers/rust,zubron/rust,graydon/rust,philyoon/rust,GBGamer/rust,vhbit/rust,AerialX/rust-rt-minimal,dwillmer/rust,aepsil0n/rust,kwantam/rust,P1start/rust,rohitjoshi/rust,dinfuehr/rust,aidancully/rust,graydon/rust,AerialX/rust-rt-minimal,jashank/rust,aepsil0n/rust,kwantam/rust,XMPPwocky/rust,aneeshusa/rust,zubron/rust,0x73/rust,nwin/rust,mvdnes/rust,dwillmer/rust,zachwick/rust,jashank/rust,quornian/rust,omasanori/rust,hauleth/rust,bombless/rust,0x73/rust,ruud-v-a/rust,krzysz00/rust,TheNeikos/rust,mahkoh/rust,GBGamer/rust,quornian/rust,krzysz00/rust,graydon/rust,avdi/rust,mihneadb/rust,graydon/rust,aneeshusa/rust,seanrivera/rust,reem/rust,GBGamer/rust,AerialX/rust,aidancully/rust,ejjeong/rust,philyoon/rust,sae-bom/rust,emk/rust,ebfull/rust,defuz/rust,zaeleus/rust,ejjeong/rust,kwantam/rust,0x73/rust,jroesch/rust,carols10cents/rust,rprichard/rust,kwantam/rust